Systems and methods for measurement or analysis of a specimen using separated spectral peaks in light

ABSTRACT

A system configured for measurement of a specimen is provided. The system includes an optical subsystem configured to perform measurements of the specimen. The optical subsystem includes a light source that is configured to generate light having a relatively large number of separated spectral peaks with substantially no continuous background. In some embodiments, the light may include vacuum ultraviolet light, extreme ultraviolet light, and/or soft x-rays. A carrier medium is also provided that includes program instructions executable on a computer system to analyze data generated by a detector of an optical subsystem by partitioning the data into individual peaks spaced apart across a wavelength spectrum. Partitioning the data preferably corrects for spectrum shift, drift, stretching, shrinking, or a combination thereof at the detector. The individual peaks correspond to separated spectral peaks in light generated by a light source of the optical subsystem.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to systems and methods formeasurement or analysis of a specimen using separated spectral peaks inlight. Certain embodiments relate to a system configured for measurementof a specimen that includes a light source configured to generate lighthaving a relatively large number of separated spectral peaks withsubstantially no continuous background.

2. Description of the Related Art

Optical systems play a significant role in the manufacturing ofintegrated circuits and other semiconductor devices. For example,optical lithography tools are used to transfer a pattern from a reticleto a resist coated wafer. The patterned features on the wafer can thenbe used to form various features of integrated circuits andsemiconductor devices. In addition, optical metrology and/or inspectiontools are used for quality control purposes in semiconductormanufacturing. The capability and throughput of these optical systemscan have a significant impact on semiconductor manufacturing. Forexample, the throughput of an optical lithography or metrology and/orinspection tool has a direct impact on the throughput of a semiconductormanufacturing process (e.g., as the throughput of the tool decreases,the throughput of the process decreases). In addition, the resolution ofa lithography tool often determines the lateral dimensions of featuresof an integrated circuit. Therefore, the resolution of the lithographytool can determine other characteristics of the integrated circuit suchas the performance bin characteristics. Likewise, the resolutioncapability of an optical metrology and/or inspection tool can have asignificant impact on a semiconductor manufacturing process since theaccuracy of the optical metrology and/or inspection tool can directlyaffect how well the process is controlled.

The resolution of an optical system depends to a large extent on thewavelength of the optical system as well as other parameters such asnumerical aperture (NA). For example, as the wavelength of the opticalsystem is decreased, the optical system can image features havingsmaller and smaller dimensions thereby increasing the resolution of thesystem. Decreasing the wavelength of a system such as a lithography toolis one way for semiconductor manufacturers to image features onto awafer having smaller dimensions. Many lithography tools used insemiconductor manufacturing today are designed for use with light havinga wavelength of 248 nm. However, lithography tools that are designed foruse with light having a wavelength of 193 nm are becoming more prevalentin semiconductor research and manufacturing.

At wavelengths around 193 nm, light may be partially absorbed by water,oxygen, and air that is present in the optical path of an opticalsystem. However, absorption levels at these wavelengths are notgenerally problematic. In contrast, as the wavelength of optical systemsfalls below 190 nm, absorption of the light by water, oxygen, and aircan cause significant problems for these systems. For instance, inlithography tools designed for use at 157 nm, the amount of lightavailable for imaging a resist on a wafer may be insufficient due to theabsorption of the light by air in the lithography tool. Furthermore,many of the light sources that are able to produce light at wavelengthsless than 190 nm are relatively low intensity or power light sources.Therefore, any absorption of the light by the environment in thelithography tool can result in a severe reduction in the imagingcapability of the optical system.

To reduce the amount of light that is lost to absorption by air, somesystems can be designed to generate a vacuum in which the opticalcomponents of the systems and the specimen that is being imaged can beplaced. Since generating and maintaining a vacuum can be relativelyexpensive, however, more common methods for reducing absorption of lighthaving wavelengths less than 190 nm involve purging the housing in whichthe optical components and the specimen are placed. Purging the housingor the tool generally involves replacing the ambient environment withinthe housing or tool with relatively pure gas such as nitrogen. Thereare, however, several problems with the current methods that are used topurge optical systems. For example, currently used methods of purginggenerally involve purging a relatively large region of the tool (e.g.,the entire tool or the entire measurement chamber). In addition, purginga large region of the tool takes a significant amount of time.Therefore, purging can have a significant adverse impact on thethroughput of the optical system.

Accordingly, it would be advantageous to develop systems and methods foroptical tools that are designed to use light that is at least partiallyabsorbed by air and that have more efficient purging systems than thosedescribed above.

SUMMARY OF THE INVENTION

An embodiment of the invention relates to a system for measurement of aspecimen. The system includes a first optical subsystem configured toperform first measurements of the specimen. The first optical subsystemis disposed within a purged environment during the first measurements.The system also includes a second optical subsystem configured toperform second measurements of the specimen. The second opticalsubsystem is disposed within a non-purged environment during the secondmeasurements.

In one embodiment, the first optical subsystem is configured to performthe first measurements using vacuum ultraviolet (VUV) light. In anotherembodiment, the second optical subsystem is configured to perform thesecond measurements using non-vacuum ultraviolet (non-VUV) light. Inother embodiments, the first optical subsystem may be configured toperform the first measurements using VUV light and non-VUV light. Insuch an embodiment, the first optical subsystem may include one or morefilters configured to prevent a substantial amount of the VUV light fromreaching the specimen during measurements with the non-VUV light.

In some embodiments, if the first optical subsystem is configured toperform the first measurements using VUV light, the first measurementsmay include a thin film measurement. In one such embodiment, if thesecond optical subsystem is configured to perform the secondmeasurements using non-VUV light, the second optical subsystem may beconfigured as a scatterometer. In additional embodiments, the firstoptical subsystem may be configured as a reflectometer, and the secondoptical subsystem may be configured as an ellipsometer. In analternative embodiment, the first optical subsystem may be configured asan ellipsometer, and the second optical subsystem may be configured as areflectometer.

In another embodiment, the first optical subsystem is configured as aspectroscopic ellipsometer or a spectroscopic reflectometer. In such anembodiment, the second optical subsystem may be configured as a singlewavelength optical subsystem. In a different embodiment, the firstoptical subsystem may include an excimer light source configured togenerate light having a wavelength of about 157 nm. According to onesuch embodiment, the first optical subsystem may be configured as asingle wavelength ellipsometer. In another different embodiment, thefirst optical subsystem may be configured as a dual beamspectrophotometer. The first optical subsystem may also be configuredsuch that both channels of the dual beam spectrophotometer are purged tosubstantially the same level during the first measurements.

In some embodiments, the first optical subsystem may include reflectivefocusing optics and reflective collecting optics. Alternatively, thefirst optical subsystem may include reflective focusing optics andtransmissive collecting optics. In another alternative, the firstoptical subsystem may include transmissive focusing optics andreflective collecting optics. In additional embodiments, the firstoptical subsystem may include one or more hollow optical fibers. Forexample, the first optical subsystem may include a light source coupledto optical components by one or more hollow optical fibers. The systemmay be configured to move the optical components with respect to thespecimen to perform the first measurements at different locations on thespecimen. In such an embodiment, the light source may be substantiallystationary during movement of the optical components.

In an additional embodiment, the system may include a differentialpurging subsystem configured to provide the purged environment for thefirst optical subsystem. In some embodiments, the specimen may bedisposed within a different purged environment during the firstmeasurements. In addition, the different purged environment may have ahigher level of unwanted molecules than the purged environment. In afurther embodiment, the system may include a specimen chamber. Thespecimen chamber may be configured to open to allow the specimen to beplaced within the specimen chamber. The specimen chamber may also beconfigured to be purged after the specimen is placed within the specimenchamber and before the first measurements.

In another embodiment, the system may include a cleaning subsystemconfigured to remove contaminants from a portion of the specimen priorto the first measurements by the first optical subsystem. In someembodiments, the system may be configured to calibrate the first opticalsubsystem with data generated by the second optical subsystem. Inaddition, the system may be configured to use data generated by thefirst and second optical subsystems in combination to determine one ormore properties of the specimen. The system may be further configured asdescribed herein.

Another embodiment relates to a different system configured formeasurement of a specimen. The system includes a first optical subsystemconfigured to perform first measurements of the specimen using VUVlight. The system also includes a second optical subsystem configured toperform second measurements of the specimen using non-VUV light. In oneembodiment, the second optical subsystem may be configured as a singlewavelength ellipsometer. In a different embodiment, the second opticalsubsystem may be configured as a beam profile reflectometer. In anotherembodiment, the second optical subsystem may include a laser lightsource.

In one embodiment, the system may be configured to maintain a purgedenvironment around the first optical subsystem during the firstmeasurements. In some embodiments, the system may include a differentialpurge subsystem configured to maintain a purged environment around thefirst optical subsystem during the first measurements. In addition, thesecond optical subsystem may be disposed within a non-purgedenvironment. The system is also configured to calibrate the firstoptical subsystem with data generated by the second optical subsystem.For example, the system may include a processor that is coupled to thefirst optical subsystem and the second optical subsystem. The processormay be configured to calibrate the first optical subsystem with the datagenerated by the second optical subsystem. The system may be furtherconfigured as described herein.

An additional embodiment relates to a computer-implemented method foranalysis of a specimen. The method includes determining one or moreproperties of the specimen using first data in combination with seconddata. The first data is measured at a VUV wavelength, and the seconddata is measured at a non-VUV wavelength. The first data may be measuredwith a first optical subsystem, and the second data may be measured witha second optical subsystem. The first and second optical subsystems maybe arranged within a single system. In addition, the first opticalsubsystem may be disposed within a purged environment in the system, andthe second optical subsystem may be disposed within a non-purgedenvironment.

The one or more properties that are determined may include opticalproperties of an upper layer on the specimen, optical properties of morethan one layer on the specimen, critical dimension of a feature on thespecimen, shape parameters of the feature on the specimen, overlayoffset between two layers on the specimen, or a combination thereof. Inone embodiment, the one or more properties may be determined using oneor more algorithms. The one or more algorithms may include a geneticalgorithm, a non-linear regression algorithm, or a comparison algorithm.In some embodiments, the first and second data may include scatterometrydata. In such an embodiment, the one or more properties may bedetermined using one or more genetic algorithms.

According to an embodiment, the one or more properties may includeoptical properties of a structure on the specimen at the VUV wavelength.In such an embodiment, the second data may be used to determine athickness of the structure on the specimen. The thickness may be used incombination with the first data to determine the optical properties.According to a different embodiment, the one or more properties mayinclude an atomic concentration of one or more structures on thespecimen. In one such embodiment, the atomic concentration may bedetermined from optical properties of the one or more structures. Theoptical properties may be determined from the first data, the seconddata, or the first and second data. In a different such embodiment, theatomic concentration may be determined by comparison of the first andsecond data with reference data. The computer-implemented method mayinclude additional steps as described herein.

A further embodiment relates to another system for measurement of aspecimen. The system includes a dual channel optical subsystemconfigured to perform measurements of the specimen using VUV light. Inan embodiment, the dual channel optical subsystem may be configured as adual beam spectrophotometer. This system also includes a purgingsubsystem configured to maintain a purged environment around the dualchannel optical subsystem during the measurements. The purging subsystemis also configured to maintain the same level of purging in bothchannels of the dual channel optical subsystem. In one embodiment, thepurging subsystem may include a differential purging subsystem.

In some embodiments, the system may include one or more additionaloptical subsystems configured to perform additional measurements of thespecimen using non-VUV light. The one or more additional opticalsubsystems may be disposed within a non-purged environment. In adifferent embodiment, the system may include an additional opticalsubsystem disposed within the purged environment. The additional opticalsubsystem may be configured as a spectroscopic ellipsometer. Theadditional optical subsystem may include a VUV light flash lamp.

In another embodiment, the dual channel optical subsystem may beconfigured to perform additional measurements using non-VUV light. Inone such embodiment, the dual channel optical subsystem may include oneor more filters that are configured to prevent a substantial amount ofthe VUV light from reaching the specimen during the additionalmeasurements with the non-VUV light.

In some embodiments, the dual channel optical subsystem may includereflective focusing optics and reflective collecting optics.Alternatively, the dual channel optical subsystem may include reflectivefocusing optics and transmissive collecting optics. In a differentalternative, the dual channel optical subsystem may include transmissivefocusing optics and reflective collecting optics. According to anotherembodiment, the dual channel optical subsystem may include one or morehollow optical fibers. In additional embodiments, the system may includea cleaning subsystem configured to remove contaminants from a portion ofthe specimen prior to the measurements. This system may be furtherconfigured as described herein.

Yet another embodiment relates to a different system configured formeasurement of a specimen. The system includes two or more opticalsubsystems configured to perform measurements of the specimen using VUVlight. The system also includes a purging subsystem configured tomaintain a purged environment around the two or more optical subsystemsduring the measurements. The purging subsystem may be further configuredto maintain approximately the same level of purging in the two or moreoptical subsystems. In one embodiment, the purging system is adifferential purging subsystem.

In one embodiment, the two or more optical subsystems include abroadband reflective spectrometer and a broadband spectroscopicellipsometer. The two or more optical subsystems may also be configuredto perform additional measurements of the specimen using non-VUV light.In addition, the two or more optical subsystems may include one or morefilters that are configured to prevent a substantial amount of the VUVlight from reaching the specimen during the additional measurements withthe non-VUV light. In another embodiment, the system also includes oneor more additional optical subsystems configured to perform additionalmeasurements of the specimen using non-VUV light. In one suchembodiment, the one or more additional optical subsystems may include abeam profile ellipsometer, a beam profile reflectometer, a broadbandreflective spectrometer, or a combination thereof. The one or moreadditional optical subsystems may be disposed within a non-purgedenvironment.

In some embodiments, a first portion of the one or more additionaloptical subsystems may be disposed within the purged environment, and asecond portion of the one or more additional optical subsystems may bedisposed within a non-purged environment. In addition, the two or moreoptical subsystems and the one or more additional optical subsystems mayhave at least one common optical component, which may include the firstportion of the one or more additional optical subsystems. The system maybe further configured as described herein.

An additional embodiment relates to another system for measurement of aspecimen. The system includes an optical subsystem configured to performmeasurements of the specimen using VUV light and non-VUV light. In oneembodiment, the optical subsystem includes one or more filters that areconfigured to prevent a substantial amount of the VUV light fromreaching the specimen during the measurements with the non-VUV light.The system also includes a purging subsystem configured to maintain apurged environment around the optical subsystem during the measurements.The purging subsystem may be a differential purging subsystem.

In an embodiment, the optical subsystem may be configured as aspectroscopic ellipsometer or a spectroscopic reflectometer. In such anembodiment, the optical subsystem may also be configured as a singlewavelength ellipsometer. In another embodiment, the optical subsystemmay be configured to perform thin film measurements using the VUV light.The optical subsystem may also be configured to perform scatterometrymeasurements using the non-VUV light. In an additional embodiment, ifthe optical subsystem is configured as a single wavelength ellipsometer,the single wavelength ellipsometer may include an excimer light sourcethat is configured to generate light having a wavelength of about 157nm. In a different embodiment, if the optical subsystem may beconfigured as a spectroscopic ellipsometer, the spectroscopicellipsometer may include a flash lamp configured to generate the VUVlight.

The optical subsystem may include one or more hollow optical fibers. Insome embodiments, the optical subsystem may include a light source thatis coupled to optical components by one or more hollow optical fibers.The system may be configured to move the optical components with respectto the specimen to perform the measurements at different locations onthe specimen. The light source may be substantially stationary duringmovement of the optical components. In one embodiment, the opticalsubsystem may include reflective focusing optics and reflectivecollecting optics. Alternatively, the optical subsystem may includereflective focusing optics and transmissive collecting optics. Inanother alternative, the optical subsystem may include transmissivefocusing optics and reflective collecting optics.

In another embodiment, the specimen may be disposed within a differentpurged environment during the measurements. In one such embodiment, thedifferent purged environment may have a higher level of unwantedmolecules than the purged environment. In some embodiments, the systemmay include a specimen chamber that is configured to open to allow thespecimen to be placed within the specimen chamber. The specimen chambermay also be configured to be purged after the specimen is placed withinthe specimen chamber and before the measurements. The system may befurther configured as described herein.

Further embodiments relate to another system for measurement of aspecimen. The system includes a cleaning subsystem that is configured toremove contaminants from the specimen prior to measurement. In oneembodiment, the cleaning subsystem may include a laser-based cleaningsubsystem. The cleaning subsystem may be configured to removecontaminants from a localized area on the specimen. In a differentembodiment, the cleaning subsystem may be a heat-based cleaningsubsystem. Such a cleaning subsystem may be configured to removecontaminants from substantially an entire surface of the specimen.

The system also includes an optical subsystem that is configured toperform measurements of the specimen using VUV light. The opticalsubsystem is disposed within a purged environment. In some embodiments,the system may include a differential purging subsystem that isconfigured to provide the purged environment for the optical subsystem.However, the cleaning subsystem may be disposed within a non-purgedenvironment.

In additional embodiments, the optical subsystem may also be configuredto perform the measurements of the specimen using non-VUV light. In onesuch embodiment, the optical subsystem may include one or more filtersthat are configured to prevent a substantial amount of the VUV lightfrom reaching the specimen during the measurements with the non-VUVlight.

In one embodiment, the system may include an additional opticalsubsystem configured to perform additional measurements of the specimenusing non-VUV light. The additional optical subsystem may be disposedwithin a non-purged environment. In a different embodiment, the systemmay include an additional optical subsystem that is configured toperform additional measurements of the specimen using YUV light. Theadditional optical subsystem may be disposed within the purgedenvironment. The system may be further configured as described herein.

Some embodiments relate to another system configured for measurement ofa specimen. This system includes an optical subsystem configured toperform measurements of the specimen. The optical subsystem includes alight source configured to generate light having a relatively largenumber of separated spectral peaks with substantially no continuousbackground. In one embodiment, the light may have wavelengths of lessthan about 200 nm. For example, the light may include VUV light.Alternatively, the light may include extreme ultraviolet (EUV) light. Inanother embodiment, the light may include soft x-rays. The relativelylarge number of separated spectral peaks may include about 10 or moreseparated spectral peaks.

In some embodiments, the optical subsystem may be configured as aspectroscopic ellipsometer, a polarimeter, a reflectometer, any otheroptical measurement subsystem known in the art, or a combinationthereof. In such embodiments, the measurements may include measurementsof thin film properties. In addition, the optical subsystem may includean array detector configured to detect light returned from the specimen.The optical subsystem may be further configured as described herein.

In one embodiment, the system may also include a processor coupled tothe optical subsystem. In one such embodiment, the processor may beconfigured to process data generated by the optical subsystem byconsidering a signal from a peak in light detected by the opticalsubsystem as having a single wavelength. The single wavelength includesa known wavelength of a center of the peak. In another such embodiment,the processor may be configured to process data generated by the opticalsubsystem by summing signals for a peak in light detected by the opticalsubsystem if the peak spans more than one pixel on a detector of theoptical subsystem. In an additional embodiment, the processor may beconfigured to process data generated by the optical subsystem bypartitioning the data into individual peaks spaced apart across awavelength spectrum. The individual peaks correspond to the separatedspectral peaks of the light generated by the light source. Theseembodiments of a system configured for measurement of a specimen may befurther configured as described herein.

Other embodiments relate to a carrier medium that includes programinstructions. The program instructions are executable on a computersystem to analyze data generated by a detector of an optical subsystemby partitioning the data into individual peaks spaced apart across awavelength spectrum. The individual peaks correspond to separatedspectral peaks in light generated by a light source of the opticalsubsystem. In one embodiment, the light generated by the light sourcemay include a relatively large number of the separated spectral peakswith substantially no continuous background. The optical subsystem isconfigured to perform measurements of a specimen. In another embodiment,the optical subsystem may be configured as a spectroscopic ellipsometer,a polarimeter, a reflectometer, any other optical measurement subsystemknown in the art, or a combination thereof. The optical subsystem may befurther configured as described herein. In some such embodiments, themeasurements may include measurements of thin film properties.

Partitioning of the data may maintain a wavelength calibration of thedetector. Partitioning of the data may also increase an accuracy of themeasurements of the optical subsystem that are performed as a functionof wavelength. In addition, partitioning of the data may correct forspectrum shift, drift, stretching, shrinking, or a combination thereofat the detector.

In some embodiments, the program instructions may be further executableon the computer system to analyze the data by summing signals for one ormore of the individual peaks if the one or more of the individual peaksspan more than one pixel on the detector. In a different embodiment, theprogram instructions may be executable on the computer system to analyzethe data by considering a signal from one or more of the individualpeaks as having a single wavelength. The single wavelength includes aknown wavelength of a center of the one or more of the individual peaks.

The light generated by the light source may have wavelengths of lessthan about 200 nm. For example, the light generated by the light sourcemay include VUV light. In another example, the light generated by thelight source may include EUV light. In a further example, the lightgenerated by the light source may include soft x-rays. The embodimentsof the carrier medium described above may be further configured asdescribed herein. For example, the carrier medium may include additionalprogram instructions executable on the computer system to performadditional computer-implemented methods described herein.

Additional embodiments relate to a method for measurement of a specimen.The method includes measuring spectroscopic ellipsometric data of thespecimen. In one embodiment, the spectroscopic ellipsometric data may bemeasured at wavelengths from about 220 nm to about 900 nm. In adifferent embodiment, the spectroscopic ellipsometric data may bemeasured at wavelengths from about 190 nm to about 300 nm. In anotherdifferent embodiment, the spectroscopic ellipsometric data may bemeasured at VUV wavelengths.

The method also includes determining a nitrogen concentration of anitrided oxide gate dielectric formed on the specimen from thespectroscopic ellipsometric data. In one embodiment, the spectroscopicellipsometric data may be measured at multiple locations on thespecimen. In one such embodiment, the method may include determining thenitrogen concentration at the multiple locations and determining awithin wafer (WIW) uniformity of the nitrogen concentration. In anotherembodiment, the method includes determining a thickness and an index ofrefraction of the nitrided oxide gate dielectric using the spectroscopicellipsometric data. In some embodiments, the method may includedetermining an index of refraction of the nitrided oxide gate dielectricfrom the spectroscopic ellipsometric data and determining the nitrogenconcentration from the index of refraction.

In an additional embodiment, the method may also include measuringreflectometric data of the specimen. In such an embodiment, determiningthe nitrogen concentration may include determining the nitrogenconcentration from the spectroscopic ellipsometric data in combinationwith the reflectometric data. The spectroscopic ellipsometric data andthe reflectometric data may be measured with one system. In someembodiments, the method may include removing contaminants from alocalized area on the specimen using a laser-based cleaning subsystemprior to measurement of the spectroscopic ellipsometric data. Thespectroscopic ellipsometric data may be measured at the localized area.

In one embodiment, measuring the spectroscopic ellipsometric data may beperformed during a semiconductor fabrication process. In anotherembodiment, the method may include altering one or more parameters of asemiconductor fabrication process based on the nitrogen concentration.In an additional embodiment, the method may include monitoring one ormore parameters of a semiconductor fabrication process using thenitrogen concentration. The method may include any additional steps ofany methods described herein.

Other embodiments relate to a computer-implemented method for analysisof a specimen. The computer-implemented method includes determining anitrogen concentration of a nitrided oxide gate dielectric formed on thespecimen from spectroscopic ellipsometric data generated by measurementof the specimen. In one embodiment, the spectroscopic ellipsometric datamay be measured at wavelengths from about 220 nm to about 900 nm.Alternatively, the spectroscopic ellipsometric data may be measured atwavelengths from about 190 nm to about 300 nm. In another alternative,the spectroscopic ellipsometric data may be measured at VUV wavelengths.In some embodiments, the spectroscopic ellipsometric data may bemeasured at multiple locations on the specimen. In such embodiments, themethod may also include determining the nitrogen concentration at themultiple locations and determining a WIW uniformity of the nitrogenconcentration.

In one embodiment, the method may include determining a thickness and anindex of refraction of the nitrided oxide gate dielectric using thespectroscopic ellipsometric data. In another embodiment, the method mayinclude determining an index of refraction of the nitrided oxide gatedielectric from the spectroscopic ellipsometric data and determining thenitrogen concentration from the index of refraction. In otherembodiments, determining the nitrogen concentration may includedetermining the nitrogen concentration from the spectroscopicellipsometric data in combination with reflectometric data generated bymeasurement of the specimen.

Additional embodiments of the method may include determining one or moreparameters of a semiconductor fabrication process based on the nitrogenconcentration. In one embodiment, the method may also include monitoringone or more parameters of a semiconductor fabrication process using thenitrogen concentration. The computer-implemented method may also includeany other steps of any method described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of a system configured for measurement of a specimen;

FIG. 2 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of a differential purging subsystem configured toprovide a purged environment for an optical subsystem;

FIG. 3 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of a system configured for measurement of a specimen,in which the specimen is disposed within a purged environment during themeasurements;

FIG. 4 is a schematic diagram illustrating a cross-sectional top view ofone embodiment of a specimen chamber;

FIG. 5 is a schematic diagram illustrating a cross-sectional side viewof the specimen chamber of FIG. 4;

FIG. 6 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of an optical subsystem that includes a VUV lightsource and a non-VUV light source;

FIG. 7 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of an optical subsystem that includes reflectivefocusing optics and reflective collecting optics;

FIG. 8 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of an optical subsystem that includes transmissivefocusing optics and reflective collecting optics;

FIG. 9 is a schematic diagram illustrating a cross-sectional side viewof one embodiment of an optical subsystem that includes reflectivefocusing optics and transmissive collecting optics;

FIGS. 10 and 11 are schematic diagrams illustrating cross-sectional sideviews of one embodiment of an optical subsystem that includes a hollowfiber;

FIG. 12 is a schematic diagram illustrating a cross-sectional side viewof an embodiment of an optical subsystem that includes a flat mirror anda substantially stationary light source;

FIG. 13 is a schematic diagram illustrating a cross-sectional side viewof an embodiment of a dual channel optical subsystem and a purgedenvironment around the dual channel optical subsystem;

FIG. 14 is a schematic diagram illustrating a cross-sectional side viewof the dual channel optical subsystem of FIG. 13 and an additionaloptical subsystem, both disposed within a purged environment;

FIGS. 15 and 16 are schematic diagrams illustrating cross-sectional sideviews of different embodiments of two of more optical subsystemsdisposed within a purged environment;

FIGS. 17 and 18 are schematic diagrams illustrating cross-sectional sideviews of different embodiments of a system configured for measurement ofa specimen that includes a cleaning subsystem and an optical subsystem;and

FIG. 19 is a plot illustrating a spectrum of a Penning discharge with amagnesium cathode and neon buffer gas.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description generally relates to optical systemsconfigured to operate at wavelengths which advantageously use a purgedenvironment. By purging only those optical subsystems that actuallybenefit from purging, cost can be saved, and reliability can beimproved. The embodiments described herein could be very important toeconomically manufacturing systems that operate at relatively lowwavelengths (e.g., vacuum ultraviolet or near vacuum ultravioletwavelengths). For example, the alternative is to enclose the wholesystem in a chamber filled with an inert gas (or vacuum) with a loadlock to allow loading and unloading of specimens without introducing toomuch oxygen, water, carbon dioxide, etc. Such an approach results inslower wafer transfer, more restricted access to the optics foralignment, and makes the system more expensive.

As used herein, the term “specimen” generally refers to a wafer or areticle. As used herein, the term “wafer” generally refers to substratesformed of a semiconductor or non-semiconductor material. Examples ofsuch a semiconductor or non-semiconductor material include, but are notlimited to, monocrystalline silicon, gallium arsenide, and indiumphosphide. Such substrates may be commonly found and/or processed insemiconductor fabrication facilities.

A wafer may include only the substrate such as a virgin wafer.Alternatively, a wafer may include one or more layers formed upon asubstrate. For example, such layers may include, but are not limited to,a resist, a dielectric material, and a conductive material. A resist mayinclude any material that may be patterned by an optical lithographytechnique, an e-beam lithography technique, or an X-ray lithographytechnique. Examples of a dielectric material may include, but are notlimited to, silicon dioxide, silicon nitride, silicon oxynitride, andtitanium nitride. Additional examples of a dielectric material include“low-k” dielectric materials such as Black Diamond™ which iscommercially available from Applied Materials, Inc., Santa Clara,Calif., and CORAL™ commercially available from Novellus Systems, Inc.,San Jose, Calif., “ultra-low k” dielectric materials such as “xerogels,”and “high-k” dielectric materials such as tantalum pentoxide. Inaddition, examples of a conductive material include, but are not limitedto, aluminum, polysilicon, and copper.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies having repeatablepattern features. Formation and processing of such layers of materialmay ultimately result in completed semiconductor devices. As such, awafer may include a substrate on which not all layers of a completesemiconductor device have been formed or a substrate on which all layersof a complete semiconductor device have been formed. The term“semiconductor device” is used interchangeably herein with the term“integrated circuit.”

The specimen may further include at least a portion of a thin-film headdie formed on a wafer, at least a portion of a micro-electro-mechanicalsystem (MEMS) device formed on a wafer, flat panel displays, magneticheads, magnetic and optical storage media, and at least a portion ofother components that may include photonics and optoelectronic devicessuch as lasers, waveguides and other passive components processed onwafers, print heads, and bio-chip devices processed on wafers.

A “reticle,” or a “mask,” is generally defined as a substantiallytransparent substrate having substantially opaque regions formed thereonand configured in a pattern. The substrate may include, for example, aglass material such as quartz. A reticle may be disposed above aresist-covered wafer during an exposure step of a lithography processsuch that the pattern on the reticle may be transferred to the resist.For example, substantially opaque regions of the reticle may protectunderlying regions of the resist from exposure to an energy source.

As used herein, the term “lithography system” generally refers to anylithography system that prints images of a reticle onto a wafer usinglight. The lithography system may be a scanning projection system or astep and scan system, which are both commonly referred to as a “scanner”or a step-and-repeat system, also called a “stepper.” The lithographysystem may include any exposure system known in the art such as systemscommercially available from Nikon, ASM Lithography, Canon, or IntegratedSolutions, Inc. The terms “lithography system,” “scanner,” and “stepper”are used interchangeably herein.

As used herein, the term “vacuum ultraviolet light” or “VUV light”refers to ultraviolet light that will be significantly absorbed by air,oxygen, carbon dioxide, and water molecules. VUV light generallyincludes light having a wavelength of less than about 190 nm. The term“non-vacuum ultraviolet light” or “non-VUV light” refers to deepultraviolet light, ultraviolet light, visible light, infrared light, orany combination thereof. Generally, the term non-VUV light refers to anylight having a wavelength greater that about 190 nm. In addition, theterm “near vacuum ultraviolet light” or “near VUV light” is used torefer to light having a wavelength of about 193 nm (e.g., about 190 nmto about 200 nm), which is partially transmissive in the atmosphere. VUVlight, non-VUV light, and near VUV light may be monochromatic, nearmonochromatic, polychromatic, or broadband light.

As used herein, the term “measurement” refers to any measurements thatmay be performed on a wafer, reticle, or other specimen. For example,measurements may include thin film measurements, measurement of alateral dimension of a layer or feature on the wafer, detection ofdefects on the wafer (i.e., inspection), and determination of one ormore properties of the defects on the wafer. In addition, themeasurements may include any other measurements of a wafer, reticle, orother specimen known in the art.

As used herein, the term “reference data” refers to data measured on areference specimen that includes one or more layers having a knownatomic concentration.

As used herein, the term “structure” refers to a layer, interface,feature, or region of a specimen.

The terms “first” and “second” as used herein are not to be construed assequential or temporal indicators. Instead, the terms first and secondare used to identify different subsystems, measurements, data, etc.

Turning now to the drawings, it is noted that FIGS. 1–18 are not drawnto scale. In particular, the scale of some of the elements of thefigures are greatly exaggerated to emphasize characteristics of theelements. It is also noted that FIGS. 1–18 are not drawn to the samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

FIG. 1 illustrates one embodiment of a system configured for measurementof specimen 10. The system includes stage 12 upon which specimen 10 isdisposed during measurement. Stage 12 may be a vacuum chuck, anelectrostatic chuck, or any other device configured to hold the specimenin place during measurement. The stage may be coupled to a mechanicalassembly (not shown). The mechanical assembly may be configured to movethe stage and thereby the specimen such that measurements can beperformed in different locations on the specimen. The mechanicalassembly may be configured to rotate the specimen during measurement.The mechanical assembly may also be configured to translate the specimenlaterally. The mechanical assembly may be configured to rotate andtranslate the specimen at the same time such that the light beam isscanned over the specimen in a spiral-like path. A stage coupled to sucha mechanical assembly may be commonly referred to as an “rθ stage.” Insome embodiments, the speeds at which the specimen is rotated andtranslated may be varied during scanning to maintain a substantiallyconstant scanning speed. In other embodiments, the mechanical assemblymay be configured to translate the specimen in two lateral directions, Xand Y. A stage coupled to such a mechanical assembly may be commonlyreferred to as an “xy stage.” In such embodiments, the mechanicalassembly may be configured to translate the specimen such that the lightbeam is scanned over the specimen in a serpentine-like path. Themechanical assembly may include any suitable mechanical or roboticassembly known in the art. In some embodiments, the stage may include anedge handling mechanism. For example, the stage may include mechanicalfeatures (not shown) that are configured to contact the edge of thespecimen and to support the specimen a spaced distance above the uppersurface of the stage. In this manner, cross-contamination of thebackside of the specimens may be reduced, and even prevented.

The system includes a first optical subsystem that is configured toperform first measurements of specimen 10. The first optical subsystemis disposed within purged environment 14 during the first measurements.In one embodiment, the first optical subsystem may be disposed inhousing 16. The housing may include any suitable housing known in theart, and many different types are commercially available. Preferably,the housing may be selected or formed to have the smallest internal areawhile allowing sufficient space for the first optical subsystem. In thismanner, the amount of time it takes to purge the housing may be kept toa minimum. In addition, the area within the system, but outside ofhousing 16 is a non-purged environment. Therefore, the amount of timethat would otherwise be required to purged the entire measurementchamber or overall housing of the system is eliminated. Although housing16 is shown in FIG. 1 to have a generally rectangular shape, it is to beunderstood that the housing may have any shape such as a cylindrical oran irregular shape.

As shown in FIG. 1, the housing includes an opening or aperture 18(referred to herein as a “differential aperture”) to allow the lightfrom the first optical subsystem to impinge upon the specimen and toallow the light returned from the specimen to be collected or detectedby the first optical subsystem. Such a differential aperture may beincluded in the housing if the light of the first optical subsystem willbe absorbed by materials of the housing. Alternatively, the housing maybe fitted with relatively small sections of a material that will notstrongly absorb or otherwise alter the light of the first opticalsubsystem. However, since such materials can be relatively expensive(particularly for the wavelength(s) of the first optical subsystemdescribed below), it may be preferable to provide a differentialaperture in the housing. The differential aperture may have any shape,but its dimensions are preferably kept as small as possible therebyreducing the amount of ambient molecules or gases that migrate into thehousing through the differential aperture. In addition, other measuresmay be taken to reduce the amount of ambient molecules or gases thatflow through the differential aperture. For example, the distancebetween the lower surface of the housing and the specimen may be reducedor minimized.

A purging subsystem (not shown in FIG. 1) may be coupled to the housing.The purging subsystem may be configured to flow dry nitrogen (N₂) oranother suitable dry inert gas into the housing such that the dry gasreplaces the ambient gas in the housing. The purging subsystem may alsobe configured to flow sufficient dry N₂ into the housing at a flow ratethat reduces the amount of ambient molecules or gases that flow throughthe differential aperture. In addition, the purging subsystem may beconfigured to purge the housing with a substantially clean gas. Forexample, the purging subsystem may include a filtration systemconfigured to filter contaminants from the dry inert gas before it isintroduced into the housing. Such filtration systems are known in theart.

Housing 16 may be purged prior to measurements by the first opticalsubsystem while at times during which no measurements are to beperformed, the environment within the housing may be non-purged orambient. Alternatively, housing 16 may be continually purged by thepurged subsystem, which may be suitable for applications such asmanufacturing where the time in which the housing is purged may reducethe throughput of the manufacturing process.

Preferably, the purging subsystem is configured to provide a purgedenvironment for the first optical subsystem that is substantially freeof water, air, oxygen, carbon dioxide, and other absorbing molecules(collectively referred to herein as “unwanted molecules”). Such purgingsubsystems are generally known in the art, and although one particularlysuitable purging subsystem will be described further below, it is to beunderstood that the purging subsystem may include any suitablecommercially available purging subsystem. The purging subsystempreferably removes the unwanted molecules from the environment in whichthe first optical subsystem is disposed because at least some of thewavelengths at which the first optical subsystem operates wouldotherwise be substantially absorbed by these unwanted molecules. Forexample, the first optical subsystem may be configured to perform thefirst measurements of the specimen using at least some wavelengths ofVUV light. In particular, the first optical subsystem may be configuredto perform the first measurements of the specimen using VUV light or VUVlight in addition to non-VUV light.

In addition, as described above, the distance between the housing andthe specimen may be reduced or kept at a minimum to reduce the amount ofunwanted molecules that flow into the housing through the differentialaperture. However, keeping the distance between the housing and thespecimen at a minimum also reduces the distance that the light of thefirst optical subsystem travels outside of the housing. Therefore,keeping this distance at a minimum will minimize the amount ofabsorption that light of the first optical subsystem experiences outsideof the housing due to the ambient environment. Other elements shown inFIG. 1 are described in further detail below.

FIG. 2 illustrates one embodiment of a purging subsystem that may beused to provide the purged environment for the first optical subsystem.This purging subsystem is similar to the multiple stage, non-contactvacuum seal described in U.S. patent application Ser. No. 10/052,307entitled “Scanning Electron Microscope Architecture and Related MaterialHandling System” filed on Jan. 17, 2002, by Koch et al., which isincorporated by reference as if fully set forth herein. However, in theembodiment shown in FIG. 2, multiple stage, non-contact seal 20 is usedto provide a purged, non-vacuum environment. In this embodiment, seal 20includes four concentric metal rings 22, 24, 26, and 28. In thisembodiment, the rings are fabricated from anodized aluminum (Al₂O₃),although other materials such as titanium could be used. Properattachment of these rings to the housing ensures that their lowersurfaces 30 are coplanar within approximately one micron.

Purging stage 20 has four purging stages. Stage 32, the outer stage, ispurged with dry N₂ or another dry gas but contains the highest levels ofunwanted molecules of the four purging stages. Stage 34 is purged tohave levels of unwanted molecules that are lower than those contained instage 32. Likewise, stage 36 is purged to have levels of unwantedmolecules that are lower than those contained in stage 34. Stage 38, thefourth and innermost stage, is purged to a level such that this stage issubstantially free of unwanted molecules. Stages 32, 34, 36, and 38 maybe individually coupled to separate assemblies 44, 46, 48, and 50,respectively. The assemblies may include pumps, tubing, filter, andother hardware suitable to independently provide the purged environmentswithin each of the stages. The assemblies can have any configurationknown in the art, and such hardware is commercially available for theseapplications.

Overpressure vent 40 creates a ring of high pressure dry gas around theentire set of seals. The overpressure prevents moisture from migratinginto the inner purging zones. Ambient pressure vent 42 creates a ring ofambient air pressure between the overpressure created by vent 40 andfirst purging stage 32. The ambient pressure zone prevents the highpressure gas discharged from vent 40 from migrating further inwardtowards the purging zones. Overpressure vent 40 and ambient pressurevent 42 may be individually coupled to separate assemblies 52 and 54,respectively. These assemblies may be configured as described above.

Although four purging stages are used in this embodiment, nothing hereinshould be taken to restrict the present invention to only using fourstages. Fewer stages could be used if a greater pressure differentialbetween each stage is acceptable although larger pumps might be requiredfor each stage. Similarly, more stages could be used, but with anassociated disadvantage of increased mechanical complexity, as well asincreased weight. The shape of the seals is also not definitive of thisembodiment. Although circular seals are easier to fabricate than someother shapes and match the shape of specimens such as semiconductorwafers, nothing herein requires that the seals be circular. Square,rectangular, or even irregularly shaped seals could be used withoutdeparting from the teachings of the present invention. The seals mayalso be used to match the shape of the housing and/or the opticalsubsystem contained within the purged environment. Multiple stage,non-contact seal 20 may be further configured as described in U.S.patent application Ser. No. 10/052,307.

Another example of a suitable purging subsystem is illustrated in U.S.patent application Ser. No. 10/718,126 entitled “Optical System forMeasuring Samples Using Short Wavelength Radiation” filed Nov. 19, 2003by Nikoonahad et al., which is incorporated by reference as if fully setforth herein. In this patent application, a method and apparatus aredisclosed in which a differential seal (somewhat analogous to the sealdescribed above, but preferably using a purge instead of vacuum) ismaintained around the optical subsystem. The purged environment is usedto prevent contamination of the optics and/or to avoid problemsassociated with dissociation of oxygen containing species due toexposure by short wavelength sources. The systems described herein aredifferent from the apparatuses described by Nikoonahad et al., in thatthe purging subsystem is used to provide a purged environment for onlysome of the optical subsystems in a tool.

In another embodiment, the optics of the first optical subsystem isdisposed in a purged environment with substantially low levels ofunwanted molecules, with only one or a few differential aperturesbetween the optics environment and the specimen environment, while thespecimen environment is also enclosed but has higher levels of unwantedmolecules than around the optics, and substantially lower levels of suchmolecules than the atmosphere. For example, the specimen may also bedisposed within a purged environment during first measurements by thefirst optical subsystem. The purged environment in which the specimen isdisposed may be different than the purged environment in which the firstoptical subsystem is disposed. For example, as shown in FIG. 3, thespecimen may be disposed within purged environment 56. Purgedenvironment 56 may be provided by a purging subsystem that includesassembly 58. The assembly may be configured as described above.

In some embodiments, unlike the first optical subsystem, the specimenmay not be disposed within a housing. Instead, a dry N₂ or other dryinert gas may be flowed across the specimen, particularly in the areabetween the specimen and the first optical subsystem. Such aconfiguration may be suitable since the purged environment in which thespecimen is disposed may have a higher level of unwanted molecules thanthe purged environment of the first optical subsystem. Such relaxedenvironmental conditions for the purged environment of the specimen maybe sufficient since the distance between the first optical subsystem andthe specimen is relatively short thereby providing less opportunity forthe light of the first optical subsystem to be absorbed.

In another embodiment, the system may include a specimen chamber inwhich the specimen may be disposed during measurement. One embodiment ofa specimen chamber is illustrated in FIGS. 4 and 5. As shown in thecross-sectional top view of FIG. 4, the specimen chamber may includehousing 60. Stage 62 is disposed within housing 60. The stage may beconfigured as described above. The specimen chamber is configured toopen to allow a specimen to be placed within the specimen chamber. Forexample, as shown in the cross-sectional side view of FIG. 5, housing 60may include door 64 coupled to side 66 of the housing. Door 64 may belowered (or otherwise moved) away from opening 68 in side 66 of housing60. Specimen 10 may then be moved through opening 68 by specimen handler70. The specimen handler may include any suitable specimen handler knownin the art such as a robotic wafer handler or other mechanical ormotorized assembly. The specimen handler may place the specimen ontostage 62. The specimen handler may then be moved out of the housing, andthe door of the housing may be moved over the opening to thereby enclosethe specimen with housing. The specimen chamber may then be purgedbefore the first measurements by the first optical subsystem. Therefore,a simple door may separate the specimen environment from the ambientenvironment allowing significant amounts of air to enter the housingwhen the specimen is loaded, which may be quickly purged to removeunwanted molecules to relatively low levels once the door is closed.Such an embodiment avoids the cost, space, and complexity of a full loadlock but still keeps relatively low levels of unwanted molecules aroundthe specimen and still lower levels of unwanted molecules around theoptics.

The specimen chamber may be configured in a number of ways such thatmeasurements of the specimen may be performed while the specimen isdisposed within the specimen chamber. In one embodiment, upper surface72 of the specimen chamber shown in FIG. 5 may be formed of a materialthat is optically transparent to light from the first optical subsystemas well as any other optical subsystems of the system. Since suchmaterials may be relatively expensive for at least some of thewavelengths (e.g., the VUV wavelengths), in another embodiment, anopening or differential aperture (not shown) may be formed in uppersurface 72 through which light from the first optical subsystem as wellas any other optical subsystems of the system may pass. In some suchembodiments, the differential aperture may have lateral dimensions thatare approximately the same as the specimen. Alternatively, thedifferential aperture may have lateral dimensions that are much smallerthan the lateral dimensions of the specimen, but are sufficient to allowlight from at least the first optical subsystem to pass through theopening. In such an embodiment, the differential aperture may be movedin conjunction with scanning of the specimen by the first opticalsubsystem. For example, the differential aperture may be formed in aplate or relatively flat sheet of material that is moved in accordancewith scanning by the first optical subsystem. However, since the amountof unwanted molecules in the purged environment around the specimen maybe relatively high, and since having a differential aperture in theupper surface of the housing that has approximately the same lateraldimensions as the specimen would allow for a simpler configuration, sucha configuration for the housing may be preferable.

Turning back to FIG. 1, FIG. 1 illustrates one embodiment of the firstoptical subsystem. In this embodiment, the first optical subsystem isconfigured as an ellipsometer. The ellipsometer includes light source74. A variety of different light sources can be used to generate VUVwavelengths for the first optical subsystem. In one preferredembodiment, a deuterium (D₂) lamp is used. Typically, the lamp will havea window made of a material such as magnesium fluoride (MgF₂) thattransmits well as such wavelengths. Excimer or high-intensity gasdischarge lamps (such as those described in U.S. Pat. No. 6,052,401 toWieser et al., U.S. Pat. No. 6,282,222 to Wieser et al., and U.S. Pat.No. 6,400,089 to Salvermoser et al., which are incorporated by referenceas if fully set forth herein) could be used as narrow band light sourcesfor use in an ellipsometer or reflectometer configuration including amulti-angle ellipsometer or a beam profile reflectometer. A flash lamp(filled with Xenon or other gas) or a Xenon arc lamp can also generatelight at VUV wavelengths.

Light source 74 may be a monochromatic or near monochromatic lightsource. For example, the first optical subsystem may include an excimerlight source configured to generate light having a wavelength of about157 nm or another VUV wavelength. In such an embodiment, theellipsometer may function as a single wavelength ellipsometer.Alternatively, light source 74 may be a polychromatic or broadband lightsource. For example, the first optical subsystem may include a flashlamp configured to generate light having VUV wavelengths. In thisembodiment, the ellipsometer may function as a spectroscopicellipsometer.

Light from light source 74 is directed to polarizer 76. In oneembodiment, the polarizer may be a Rochon prism. In some embodiments,the polarizer may rotate such that the ellipsometer is configured as arotating polarizer ellipsometer (RPE). Light passing through thepolarizer is directed onto specimen 10.

Light reflected from the specimen is collected by analyzer 78. Analyzer78 may include any polarizing element known in the art. In a RPEconfiguration, the position of the analyzer is fixed. However, in someembodiments, both polarizer 76 and analyzer 78 may rotate. One exampleof an ellipsometer system that includes a rotating polarizer and arotating analyzer is disclosed in International Publication No. WO00/65331 by Wang et al., which is incorporated by reference as if fullyset forth herein. In a spectroscopic ellipsometer configuration, thefirst optical subsystem may include spectrometer 80 and detector 82. Thespectrometer or another dispersion element may be configured to separatebeam components having different wavelengths. The separated componentsof the beam may be detected by individual elements of an array of thedetector. In a non-spectroscopic ellipsometer configuration, the firstoptical subsystem may include detector 82, but not spectrometer 80.Detector 82 may include any appropriate detector known in the art.

Although detector 82 is shown to be located within the purgedenvironment contained within housing 16, it is to be understood that thedetector and any associated electronics may alternatively be locatedoutside of housing 16 and therefore outside of the purged environmentfor the first optical subsystem. In such an embodiment, a seconddifferential aperture (not shown) may be formed within the housing.Light from the spectrometer may be directed through the additionaldifferential aperture to the photosensitive elements of the detector.Placing the detector and any associated electronics outside of thepurged environment within housing 16 may allow the area within thehousing to be reduced thereby reducing the time it takes to create thepurged environment and the cost of creating and maintaining the purgedenvironment.

Although one particular ellipsometer configuration is shown in FIG. 1,it is to be understood that the ellipsometer may have any configurationknown in the art. For example, the first optical subsystem may beconfigured as a rotating compensator ellipsometer (RCE), which is knownin the art, or a beam profile ellipsometer, which is described furtherherein. In addition, the ellipsometer may be configured as a variableangle ellipsometer. Further examples of ellipsometer configurations areillustrated in U.S. patent application Ser. No. 10/056,271 entitled“Laser-Based Cleaning Device For Film Analysis Tool,” by Janik et al.and U.S. Pat. No. 5,166,752 to Spanier et al., U.S. Pat. No. 5,608,526to Piwonka-Corle et al., U.S. Pat. No. 5,910,842 to Piwonka-Corle etal., and U.S. Pat. No. 6,515,746 to Opsal et al., all of which areincorporated by reference as if fully set forth herein. Such systems, ifconfigured for use at VUV wavelengths, can also benefit from the purgingconfigurations described herein.

In one embodiment, the purged environment around the optics and thedifferential aperture may be used to improve the signal level formeasurements made at wavelengths that are partially transmissive in theatmosphere (and thus cannot be strictly called VUV) such as 193 nm ornear VUV wavelengths. Because there is some absorption of light atwavelengths near 193 nm, useful improvements in the signal level atthese wavelengths can be obtained by purging all, or part of, the lightpath.

In some embodiments, the first optical subsystem may also be configuredto perform first measurements of the specimen using VUV light andnon-VUV light. In this manner, a single purged measurement light pathcan be used for both VUV and longer wavelengths. Such an opticalsubsystem may include one light source that is configured to provideboth VUV light and non-YUV light. One example of such a light source isa Xenon arc lamp. Alternatively, the first optical subsystem may includetwo light sources, one which provides VUV light and another whichprovides non-VUV light. FIG. 6 illustrates one such embodiment of afirst optical subsystem. Although this figure illustrates only a firstoptical subsystem, it is to be understood that this embodiment of thefirst optical subsystem may be incorporated in any of the systemsdescribed herein. As shown in FIG. 6, the first optical subsystemincludes light source 74 and light source 83 a. Light source 74 may beconfigured to provide VUV light. Light source 83 a may be configured toprovide non-VUV light. In such embodiments, light source 74 may includeany of the light sources described above, and light source 83 a mayinclude a white light source, a laser light source, or any other non-VUVlight source.

The VUV light and the non-VUV light may be combined into a single beamby dichroic mirror 83 b or another suitable optical component.Therefore, the VUV light and the non-VUV light may be simultaneouslydirected to specimen 10 at the same incidence angle. Alternatively, thefirst optical subsystem may be configured to direct the VUV light andthe non-VUV light to the specimen at different angles of incidence. Forexample, each of the light sources may be coupled to different focusingoptics and/or other optical components that are configured to direct thelight to the specimen. The light sources, the focusing optics, and theother optical components may be arranged such that the light from thedifferent light sources may be directed to the specimen at different,independent angles of incidence.

Such embodiments may include filter(s) that move into the light path tolimit the exposure of the specimen to VUV radiation. In one suchembodiment, the first optical subsystem may include one or more filters84, as shown in FIG. 1. Although filter(s) 84 are shown in FIG. 1 to belocated between the polarizer and the specimen, it is to be understoodthat the filter(s) may be disposed in any location in the optical pathbetween the light source and the specimen. The one or more filters maybe configured to prevent a substantial amount of the VUV light fromreaching the specimen during measurements with the non-VUV light. Aselection of filters could be used, each with successively shortercutoff wavelengths (for example, at 400 nm, 300 nm, and 190 nm) so thatthe specimen is exposed only to wavelengths needed for a particularmeasurement and no wavelengths substantially shorter than those needed.The one or more filters may include any appropriate filter(s) known inthe art such as bandpass filter(s), edge filter(s), notch filter(s), ora combination thereof.

In addition, the first optical subsystem may be configured to functionas multiple measurement subsystems. For example, the first opticalsubsystem may be configured as a spectroscopic ellipsometer as well as asingle wavelength ellipsometer. In one such embodiment, the firstoptical subsystem may include a broadband light source configured toprovide light for the spectroscopic ellipsometer. In one example, thespectroscopic ellipsometer may include a flash lamp configured togenerate the VUV light. In addition, the first optical subsystem mayinclude a monochromatic or near monochromatic light source configured toprovide light for the single wavelength ellipsometer. In one particularembodiment, the single wavelength ellipsometer may include an excimerlight source configured to generate light having a wavelength of about157 nm. Such an optical subsystem may be further configured as describedand illustrated in FIG. 6. Alternatively, the first optical subsystemmay include a broadband light source and one or more filters coupled tothe broadband light source. The filter(s) may be moved out of theoptical path during operation of the first optical subsystem as aspectroscopic ellipsometer such that multiple wavelengths are directedto the specimen. The filter(s) may be moved into the optical path duringoperation of the first optical subsystem as a single wavelengthellipsometer. Such a first optical subsystem may be further configuredas described and illustrated in FIG. 1.

The first optical subsystem may also include reflective or transmissivefocusing optics and reflective or transmissive collecting optics. Forexample, FIGS. 7–9 illustrate three different embodiments of a firstoptical subsystem. Although these figures illustrate only a firstoptical subsystem, it is to be understood that these embodiments of thefirst optical subsystem may be incorporated in any of the systemsdescribed herein. As shown in FIG. 7, the first optical subsystemincludes light source 74 and polarizer 76, which may be configured asdescribed above. The first optical subsystem also includes reflectivefocusing optics 86 configured to focus light from the polarizer tospecimen 10. The first optical subsystem may optionally include one ormore filters 84 that are configured as described above. Although the oneor more filters are shown to be placed in the optical path betweenreflective focusing optics 86 and specimen 10, it is to be understoodthat the one or more filters may alternatively be placed in any locationbetween light source 74 and specimen 10.

This embodiment of the first optical subsystem also includes reflectivecollecting optics 88. Reflective collecting optics 88 may be configuredto collect light from specimen 10 and to focus the collected light topolarizer 78, which may be configured as described above. The firstoptical subsystem may also include spectrometer 80, which may beconfigured as described above. In addition, the first optical subsystemincludes detector 82, which may be configured as described above.

Reflective focusing optics 86 and reflective collecting optics 88 mayinclude any focusing mirrors known in the art. Reflective focusingoptics and reflective collecting optics may be particularly suitable forthe VUV wavelengths of the first optical subsystem. For example,reflective optics may induce significantly less aberrations in VUV lightthan transmissive optics. In addition, reflective optics may be lessexpensive than transmissive optics that are suitable for VUV light.

However, transmissive optics may be included in the first opticalsubsystem. FIGS. 8 and 9 illustrate two such embodiments. For example,the embodiment of the first optical subsystem illustrated in FIG. 8 issimilar to that shown in FIG. 7 except that transmissive focusing optics90 are used in place of reflective focusing optics 86. Examples ofsuitable optical components for transmissive focusing optics 90 are aspherical lens, another type of lens, or a combination of lenses, eachof which may be made of calcium fluoride (CaF₂). As shown in FIG. 8,optional one or more filters 84 may be disposed in the optical pathbetween polarizer 76 and transmissive focusing optics 90. Alternatively,optional one or more filters 84 may be disposed in any location betweenlight source 74 and specimen 10. The embodiment shown in FIG. 8 alsoincludes reflective collecting optics 88, which may be configured asdescribed above. In an alternative, the embodiment of the first opticalsubsystem illustrated in FIG. 9 is similar to that shown in FIG. 7except that transmissive collecting optics 92 are used in place ofreflective collecting optics 88. Transmissive collecting optics 92 mayinclude a spherical lens, another type of lens, or a combination oflenses, each of which may be made of CaF₂. As further shown in FIG. 9,the first optical subsystem may include reflective focusing optics 86,which may be configured as described above. In some embodiments, thefirst optical subsystem may include transmissive focusing optics asshown in FIG. 8 and transmissive collecting optics as shown in FIG. 9.

In the VUV optical subsystem, the light can be transmitted from thelight source to the measurement location by any means suitable for thewavelengths involved including reflection from mirrors, transmissionthrough lenses, and transmission through hollow optical fibers. Asdescribed further herein, the use of flat mirrors or fibers can allowthe measurement head to move relative to the specimen without having tomove the light source.

In one embodiment, the first optical subsystem may include one or moreone or more hollow optical fibers. Hollow optical fibers are definedherein as optical fibers that have at least some voided space within theoptical core of the optical fibers. Hollow optical fibers may beparticularly advantageous for the transmission of VUV light of the firstoptical subsystem since the VUV light may be substantially absorbed bythe solid optical core of conventional optical fibers. Hollow opticalfibers are being developed by a number of companies and organizationsincluding, but not limited to, Bell Labs, Corning, and the University ofCalifornia at Riverside.

In one embodiment shown in FIG. 10, the first optical subsystem mayinclude hollow optical fiber 94. Hollow optical fiber 94 is configuredto direct light from light source 74 to an optical component of thefirst optical subsystem. For example, hollow optical fiber 94 may beconfigured to direct light from light source 74 to polarizer 76. Thefirst optical subsystem is shown in this embodiment to have a similarconfiguration to that shown in FIG. 7. However, a hollow optical fibermay be used to transmit light from the light source to the polarizer oranother optical component of any other embodiments of the first opticalsubsystem described herein. In some embodiments, the first opticalsubsystem may also include an additional hollow optical fiber (notshown). The additional hollow optical fiber may be configured totransmit light from the analyzer, the spectrometer or another opticalcomponent to the detector.

In some embodiments, the system may be configured to move the opticalcomponents of the first optical subsystem with respect to the specimento perform the first measurements at different locations on thespecimen. However, the light source may be substantially stationaryduring movement of the optical components. For example, as shown bycomparison between FIGS. 10 and 11, the system may be configured to movepolarizer 76, reflective focusing optics 86, one or more filters 84,reflective collecting optics 88, analyzer 78, spectrometer 80, anddetector 82 with respect to specimen 10 such that different locations onthe specimen may be measured by the first optical subsystem. However,despite the movement or different positions of these optical components,light source 74 may not be moved from its initial position. Instead,hollow optical fiber 94 may be sufficiently flexible and may have asufficient length such that the hollow optical fiber can extend from thelight source to the polarizer over a range of positions of thepolarizer.

Such an embodiment may be particularly advantageous for relatively largeor sensitive light sources such as excimer light sources since movingsuch light sources may be costly and complex. In addition, in suchembodiments, the light source may be disposed outside of the purgedenvironment within housing 16. A differential aperture in the housingmay allow the hollow optical fiber to extend from the light sourceoutside of the housing to an optical component inside the housing suchas polarizer 76. Such embodiments may be particularly advantageous sincethe area within the housing may be reduced by moving the light sourceoutside of the housing thereby reducing the area of the purgedenvironment. In a similar manner, a suitable hollow optical fiber thattransmits light from an optical component of the first optical subsystemto a detector may allow the detector to have a substantially stationaryposition during movement of the optical components. Such a configurationmay be appropriate for a detector that is located inside housing 16 oroutside of housing 16. Although FIGS. 10 and 11 illustrate only a firstoptical subsystem, it is to be understood that these embodiments of thefirst optical subsystem may be incorporated in any of the systemsdescribed herein.

In another embodiment, the first optical subsystem may include a flatmirror. One such embodiment is illustrated in FIG. 12. In thisembodiment, light from light source 74 is directed to flat mirror 96. Insome embodiments, light may be directed from the light source to theflat mirror by a hollow optical fiber (not shown). Flat mirror 96directs the light from the light source to an optical component of thefirst optical subsystem such as polarizer 76. The flat mirror may becoupled to a mechanical assembly or other mechanism (not shown) that isconfigured to alter a position of the flat mirror such that the angle atwhich light is reflected from the flat mirror is altered. For example,the position of the flat mirror may be altered bi-directionally as shownby the vector in FIG. 12.

In this embodiment, the position of the light source may besubstantially stationary. In addition, the position of flat mirror maybe fixed laterally with respect to the light source. The opticalcomponents may be moved as described above to perform measurements atdifferent locations on the specimen. To account for the differentpositions between the light source and these optical components, theposition of the flat mirror (and optionally the position of one or moreoptical components such as polarizer 76) may be altered such thatirregardless of the position of the optical components, the light fromthe light source may be directed to the appropriate optical componentsat the appropriate angles. Although the first optical subsystem is shownto include only one flat mirror in FIG. 12, it is to be understood thatthe first optical subsystem may include more than one flat mirror (notshown), each of which may be configured to reflect the light from thelight source at various angles to account for movement of the opticalcomponents. Although FIG. 12 illustrates only a first optical subsystem,it is to be understood that this embodiment of the first opticalsubsystem may be incorporated in any of the systems described herein.

Turning back again to FIG. 1, the system shown in FIG. 1 also includes asecond optical subsystem. The second optical subsystem is configured toperform second measurements of the specimen. The second opticalsubsystem is disposed within a non-purged environment (i.e., an ambientenvironment or an environment containing a substantial number ofunwanted molecules) during the second measurements. For example, asshown in FIG. 1, the second optical subsystem is disposed outside ofhousing 16, which contains the purged environment for the first opticalsubsystem. In addition, although the second optical subsystem isdisposed within a measurement chamber or overall housing (not shown) ofthe system, the measurement chamber or the overall housing is notpurged. The second optical subsystem may be disposed in a non-purgedenvironment since the second optical subsystem is configured to performthe second measurements using non-VUV light.

In the embodiment shown in FIG. 1, the second optical subsystem isconfigured as a reflectometer. For example, the second optical subsystemincludes light source 98. Light source 98 may be a monochromatic or nearmonochromatic light source. In this embodiment, the reflectometer may beconfigured as a single wavelength reflectometer. One example of anappropriate monochromatic light source is a single wavelength laserlight source. In another embodiment, the light source may be apolychromatic light source or broadband light source. In such anembodiment, the reflectometer may be configured as a spectroscopicreflectometer. One example of an appropriate broadband light source is aXenon arc lamp that is configured to emit a light beam that includesvisible and ultraviolet light.

The second optical subsystem also includes beam splitter 100. The beamsplitter is configured to direct light emitted from light source 98 tospecimen 10. The beam splitter may be a beam splitter mirror that canproduce a continuous broadband spectrum of light. The second opticalsubsystem further includes lens 102, which is configured to focus lightpropagating from beam splitter 100 onto specimen 10. Light returned fromthe specimen may pass through lens 102 and beam splitter 100 todiffraction grating 104. The diffraction grating may be configured todisperse light returned from the specimen. The dispersed light may bedirected to a spectrometer such as detector array 106. The detectorarray may include a linear photodiode array. The light may be dispersedby a diffraction grating as it enters the spectrometer such that theresulting first order diffraction beam of the sample beam may becollected by the linear photodiode array.

Although one particular configuration of a reflectometer is illustratedin FIG. 1, it is to be understood that the second optical subsystem mayhave any reflectometer configuration known in the art. Additionalexamples of spectroscopic reflectometers are illustrated in U.S. Pat.No. 4,999,014 to Gold et al. and U.S. Pat. No. 5,747,813 to Norton,which are incorporated by reference as if fully set forth herein. Thesecond optical subsystem may also be configured as a beam profilereflectometer. Such a configuration is described further herein below.

Therefore, a plurality of optical subsystems may be used in combinationin a single tool or system. However, only some of these opticalsubsystems (e.g., the first optical subsystem) are maintained in apurged environment. The purged environment can be created with adifferential seal, such as that described by Nikoonahad et al.Preferably, the optical subsystems that are kept purged are those whichuse illumination sources having short wavelengths (e.g., VUVwavelengths).

The system shown in FIG. 1 includes a first optical subsystem configuredas an ellipsometer and a second optical subsystem configured as areflectometer. The ellipsometer may be disposed in a purged environmentsuch that the ellipsometer can operate at VUV wavelengths while thereflectometer is in atmosphere and is limited to wavelengths longer thanabout 190 nm. Similar configurations could be envisioned for the manyvarious configurations described in U.S. Pat. No. 6,633,831 toNikoonahad et al., which is incorporated by reference as if fully setforth herein, and/or those combinations of subsystems present incommercially available tools from KLA-Tencor, Therma-Wave, and RudolphTechnology.

However, the system may include other combinations of opticalsubsystems. For example, in one embodiment, the first optical subsystemmay be configured as a reflectometer, and the second optical subsystemmay be configured as an ellipsometer. Such a configuration may bedesirable for as system such as the F5 system commercially availablefrom KLA-Tencor, which is described in U.S. Pat. No. 5,608,526 toPiwonka-Corle et al., which is incorporated by reference as if fully setforth herein. In particular, it might be desirable to have thereflectometer at a shorter wavelength than the ellipsometer. Therefore,it could be desirable to maintain a purged environment around just thereflectometer while permitting the ellipsometer to be exposed to anoxygen containing atmosphere.

In another example, the '752 patent referenced above describes amulti-angle ellipsometer. It is convenient to use one or more lasers aslight sources for such an ellipsometer because of the brightness andstability of most lasers. Since lasers that operate at VUV wavelengthsare expensive, in one preferred embodiment, a multi-angle ellipsometerwith one or more visible or near infra-red lasers (such as gas or diodelasers) operating in atmosphere may be combined with a reflectometerusing a VUV source such as a D₂ lamp. The reflectometer is purged withan inert gas utilizing a differential aperture.

In another embodiment, the first optical subsystem may be configured asan ellipsometer, a reflectometer, or another thin film measurementoptical subsystem, and the first or second optical subsystem may beconfigured as a scatterometer. One example of an optical subsystem thatmay be configured as both a spectroscopic ellipsometer and aspectroscopic scatterometer is illustrated in International PublicationNo. WO 99/45340 to Xu et al., which is incorporated by reference as iffully set forth herein. Such an optical subsystem may be configured asdescribed herein to perform measurements of a specimen using both VUVlight and non-VUV light. The scatterometer may also have any otherconfiguration known in the art.

The optical subsystems could be used to measure film properties andcritical dimension (CD) and other shape parameters. Different wavelengthranges may be used for determining the film properties than fordetermining CD and other shape properties. For example, it might beadvantageous to use wavelengths that extend into the VUV for opticalproperty measurements, but use only near-UV and longer wavelengths forthe CD measurements. In such an embodiment, the first optical subsystemmay be configured to perform thin film measurements using the VUV light.In addition, the first or second optical subsystem may be configured toperform scatterometry measurements using the non-VUV light. In othercases with very fine structures, it might be advantageous to use VUVwavelengths for the CD measurements, but near-UV and/or longerwavelength measurements may suffice for the determination of filmthickness and optical properties.

In yet another embodiment, the first optical subsystem may be configuredas a spectroscopic ellipsometer or a spectroscopic reflectometer, andthe second optical subsystem may be configured as a single wavelengthoptical subsystem. In a further embodiment, the first optical subsystemmay be configured as a dual beam spectrophotometer, one configuration ofwhich is described further below, and the second optical subsystem maybe configured as a different optical subsystem. In each of theseembodiments, the first optical subsystem may be disposed in a purgedenvironment, and the second optical subsystem may be disposed in anon-purged environment. In addition, the first optical subsystem may beconfigured to perform measurements at VUV wavelengths, and the secondoptical subsystem may be configured to perform measurements at non-VUVwavelengths.

In a further embodiment, the '746 patent referenced above describes ameasurement system that may incorporate up to five different measurementtechnologies (see, for example, FIG. 1 of the '746 patent). Three ofthese measurement technologies (single wavelength or “absolute”ellipsometer, beam profile reflectometer, and beam profile ellipsometer)use a laser light source operating at a wavelength that transmitsthrough the atmosphere. The other two measurement technologies (thebroadband reflectometer and the spectroscopic ellipsometer) usebroadband light sources (such as a combination of a D₂ lamp with aquartz halogen or Xenon arc lamp). In one preferred embodiment, thesystem may be altered such that one, or both, of those broadbandmeasurement technologies operates at VUV wavelengths and is contained inan inert gas environment that is separated from the atmosphere by atleast one differential aperture, and is combined with at least one othermeasurement technology that has at least part of its light path in theatmosphere. One such embodiment is illustrated in FIG. 16, which isdescribed in more detail below.

The system shown in FIG. 1 (and other system embodiments describedherein) may also include processor 108. Processor 108 is coupled to boththe first optical subsystem and the second optical subsystem bytransmission media, indicated by the dotted lines. The transmissionmedia may include wire(s), cable(s), wireless transmission path(s),network(s), or a combination thereof. The transmission media may alsoinclude “wired” and “wireless” portions. Although processor 108 is shownto be directly coupled to the first optical subsystem and the secondoptical subsystem (e.g., directly coupled to detector array 106 of thesecond optical subsystem), it is to be understood that the processor maybe indirectly coupled to the first and/or second optical subsystemsthrough one or more intermediate components such as electronics or localprocessors.

Processor 108 may perform one or more functions on data generated by thefirst and second optical subsystems. In one embodiment containingmultiple measurement technologies, one measurement technology may beused to calibrate other measurement technologies. The referencemetrology technology is used to calibrate other technologies includingthose operating in the purged environment or at VUV wavelengths. Forexample, the system may be configured to calibrate the first opticalsubsystem with data generated by the second optical subsystem. Processor108 may perform at least a portion of that calibration. If the system isconfigured to calibrate the first optical subsystem with data generatedby the second optical subsystem, then the second optical subsystem ispreferably a substantially accurate and stable subsystem. For example,in one such embodiment, the second optical subsystem may be configuredas a single wavelength ellipsometer. In an alternative embodiment, thesecond optical subsystem may be configured as a beam profilereflectometer. Such second optical subsystems may include a laser lightsource, which is advantageous for calibration because of the stabilityof the laser.

In an additional embodiment, the processor may be configured to use datagenerated by the first optical subsystem to determine one or moreproperties of the specimen. For example, an intensity of light at theelements of the detector array may be converted by the processor toellipsometric parameters, ψ and Δ, by mathematical equations known inthe art. The ellipsometric parameters may be typically shown as tan ψand cos Δ. Tan ψ is the amplitude of the complex ratio of the s and pcomponents of the reflectivity of the sample, and Δ is the phase of thecomplex ratio of the s and p components of the reflectivity of thesample. The term “s component” is used to describe the component for thepolarized radiation having an electrical field perpendicular to theplane of incidence of the reflected beam. The term “p component” is usedto describe the component for the polarized radiation having anelectrical field in the plane of incidence of the reflected beam. Forvery thin films, tan ψ may be independent of thickness, and Δ may belinearly proportional to the thickness.

Software integrated into the processor may be configured to convert theellipsometric parameters, ψ and Δ, to an optical property of a specimenusing a mathematical, or optical, model. Typically, a personal computerhaving a software package operable to rapidly performing data-fittingcalculations such as a least-squares fitting technique may beappropriate for this use. Because ellipsometric parameters including ψand Δ may be determined at small increments across a broad spectrum ofwavelengths and at several angles, several hundred data points may beincluded in the calculations. Several software packages configured foruse with spectroscopic ellipsometers that are capable of handling such alarge amount of data are commercially available.

There are several optical models that may be used to analyzeellipsometric data. Examples, of such models include, but are notlimited to, a cauchy model, a harmonic oscillator model, and apolynomial series expansion model. An appropriate model may be chosenbased on specimen characteristics, desired optical properties of thespecimen, and the computational difficulty associated with the model.For example, the cauchy model is a relatively straightforwardmathematical model. The cauchy model, however, may not be valid forwavelengths at which a specimen exhibits absorption. Additionally,optical properties of several layers of a specimen may also bedetermined simultaneously by using an appropriate optical model or acombination of optical models. Therefore, when using spectroscopicellipsometry to analyze a specimen, one or more optical models may bemore appropriate for analysis than others.

Thicknesses, indexes of refraction, and extinction coefficients (whichare commonly referred to as “thin film measurements”) for a layer of aspecimen, a portion of a layer of a specimen, or several layers of aspecimen may be determined from ellipsometric parameters using anoptical model. The index of refraction, “n,” is related to the speed oflight as it moves through a medium and is dependent upon the wavelengthof the light. The extinction coefficient, “k,” is also dependent uponwavelength and relates to absorption of light by a medium. Theextinction coefficient may also be used to determine the absorptioncoefficient for a given wavelength. Further discussion of theellipsometric parameters and the optical properties of materials isillustrated in U.S. Pat. No. 4,905,170 to Forouhi, et al., which isincorporated by reference as if fully set forth herein.

In another embodiment, the processor may be configured to use datagenerated by the second optical subsystem to determine one or moreproperties of the specimen. For example, the photodiode array ofdetector array 106 may measure the reflectance spectrum of the lightreturned from the surface of the specimen. The relative reflectancespectrum may be obtained by dividing the intensity of the returned lightof the reflectance spectrum at each wavelength by a relative referenceintensity at each wavelength. A relative reflectance spectrum may beused to determine the thickness of various films on the specimen. Inaddition, the reflectance at a single wavelength and the refractiveindex of the film may also be determined from the relative reflectancespectrum. Furthermore, a model method by modal expansion (MMME) modelmay be used to generate a library of various reflectance spectrums. TheMMME model is a rigorous diffraction model that may be used to calculatethe theoretical diffracted light “fingerprint” from each grating in theparameter space. Alternative models may also be used to calculate thetheoretical diffracted light, however, including, but not limited to, arigorous coupling waveguide analysis (RCWA) model. The measuredreflectance spectrum may be fitted to the various reflectance spectrumsin the library. The fitted data may also be used to determine a criticaldimension such as a lateral dimension, a height, and/or a sidewall angleof a feature on the specimen. Processor 108 may be configured to performall of the above calculations. Examples of modeling techniques areillustrated in International Application No. WO 99/45340 to Xu et al.,which is incorporated by reference as if fully set forth herein.

In a further embodiment, where one of the optical subsystems isconfigured as a scatterometer (e.g., a single wavelength scatterometeror a spectroscopic scatterometer), the processor may be configured touse data from the scatterometer to determine one or more properties ofthe specimen. The properties may include a critical dimension, a height,and/or a sidewall angle of a feature on the specimen. Examples of howsuch properties can be determined from scatterometry data areillustrated in International Publication No. WO 99/45340 to Xu et al.and U.S. patent application Ser. No. 09/927,102 to Weber-Grabau et al.,which are incorporated by reference as if fully set forth herein.Alternatively, or in addition, the properties may include an overlaymeasurement of the specimen. Overlay generally refers to a lateralposition of a feature on one level of a wafer with respect to a lateralposition of a feature on another level of the wafer. Examples of howoverlay can be determined from scatterometry data are illustrated inU.S. patent application Ser. No. 09/927,102 to Weber-Grabau et al. andInternational Publication No. WO 02/25723 to Brill et al., which areincorporated by reference as if fully set forth herein.

In any situation where a measurement system incorporates more than onemeasurement technology and where at least one of those technologiesoperates at a VUV wavelength, it may be advantageous to combine ameasurement made at one or more VUV wavelengths with a measurement madeat one or more non-VUV wavelengths in order to get better measurementresults than can be obtained by either wavelength ranges alone. In oneembodiment, the system may be configured to use data generated by thefirst and second optical subsystems in combination to determine one ormore properties of the specimen. The determinations may be performed bythe processor. For example, the processor may be configured to perform acomputer-implemented method for analysis of a specimen. The methodincludes determining one or more properties of the specimen using firstdata in combination with second data. The first data is measured at aVUV wavelength, and the second data is measured at a non-VUV wavelength.For example, the first data may be measured with the first opticalsubsystem, which is disposed within a purged environment in the system.The second data may be measured with the second optical subsystem. Thefirst and second optical subsystems may be disposed in a single systemas described herein, and the second optical subsystem may be disposedwithin a non-purged environment in the system. Alternatively, the firstand second data may be measured with the first optical subsystem, whichis disposed within a purged environment in the system.

There are many ways to do such combining, including using all themeasurements as constraints with appropriate relative weighting in anon-linear regression, or by using one wavelength range from onemeasurement technology to first determine one parameter such asthickness and then using another wavelength range from anothermeasurement technology to determine another parameter or parameters suchas refractive index. In addition, genetic algorithms can be used tocombine the results from multiple measurement subsystems. Many differentalgorithms can be used individually or in combination to extract theresults from the data. In one embodiment, the one or more properties maybe determined using one or more algorithms. The one or more algorithmsmay include a genetic algorithm, a non-linear regression algorithm, acomparison algorithm (e.g., comparison with a database (or library) orpre-computed or pre-measured results), or a combination thereof. Manysuch algorithms are known in the art, and the processor may use any ofthese algorithms to determine the one or more properties. Examples ofgenetic algorithms are illustrated in U.S. Pat. No. 5,953,446 to Opsalet al. and U.S. Pat. No. 6,532,076 to Sidorowich, which are incorporatedby reference as if fully set forth herein. In one embodiment, the firstand second data may include scatterometry data. In such an embodiment,it may be particularly advantageous to determine the one or moreproperties of the specimen using one or more genetic algorithms.

If it is desired to know the thickness of a film on a substrate and itsoptical properties such as refractive index, n, and absorption, k, at aVUV wavelength such as 157 nm, a better measurement result may beobtained by combining a measurement made with a visible wavelength,which might give a more accurate measurement of film thickness, with ameasurement made at VUV wavelengths rather than using just VUVwavelengths. In this manner, the second data may be used to determine athickness of a structure on a specimen. This thickness may be used incombination with the first data to determine the optical properties ofthe structure at one or more VUV wavelengths.

In a different embodiment, the one or more properties may include anatomic concentration of one or more structures on the specimen. In onesuch embodiment, the atomic concentration may be determined from opticalproperties of the one or more structures. In this manner, the atomicconcentration may be inferred indirectly from the optical properties(such as n and/or k). The optical properties may be determined from thefirst data, the second data, or the first and second data. In analternative embodiment, the atomic concentration may be determined bycomparison of the first and second data with reference data. In thismanner, the atomic concentration may be determined directly bycomparison with results from previously measured samples. Materials thatmay be of interest for atomic concentration measurements include, butare not limited to, carbon, nitrogen, oxygen, cobalt, nickel, titanium,zirconium, hafnium, tantalum, tungsten, rhenium, silicon, germanium, andvarious transition and rare-earth metals.

The one or more properties that are determined from the first and seconddata may include other properties of the specimen such as, but notlimited to, optical properties of an upper layer on the specimen,optical properties of more than one layer on the specimen, criticaldimension (CD) of a feature on the specimen, shape parameters of thefeature on the specimen, overlay offset between two layers on thespecimen, or a combination thereof.

The processor may also be configured to perform other metrology and/orinspection functions on the data generated by the first and/or secondoptical subsystems. For example, the processor may be configured todetect defects on the specimen using data from the first and/or secondoptical subsystem. The processor may detect defects on the specimenusing any algorithm or method known in the art such as a thresholdingalgorithm or a die-to-die comparison method.

Metrology methods and systems are also provided for nitrided oxide gatefilm process monitoring and process control for semiconductor devicefabrication. Several methods for nitrided oxide gate film processmonitoring and control are currently used. For example, methods that maybe used for nitrogen concentration and dose measurements include x-rayphotoemission spectroscopy (XPS) and secondary ion-emission massspectroscopy (SIMS). There are, however, several disadvantages to thecurrently used methods. In addition, this measurement technique hasrelatively poor thickness accuracy and repeatability. Furthermore, XPSis generally slow, expensive (e.g., about $300 per analysis), and notcapable of fast, in-line process monitoring and control in a high volumesemiconductor manufacturing application. Similarly, SIMS is alsodestructive in that the wafer must be broken and the film being analyzedis damaged. In addition, this measurement technique has relatively poorthickness accuracy and repeatability and is not suitable for high volumesemiconductor manufacturing applications.

Embodiments of a method for measurement of a nitrided oxide gatedielectric described herein may or may not include forming a nitridedoxide gate dielectric on a specimen. The specimen in such embodiments isa wafer. In general, a nitrided oxide gate dielectric is formed on awafer by exposing an oxygen-containing layer such as silicon dioxide toa nitrogen-containing plasma. Examples of methods for forming a nitridedoxide on a wafer are illustrated in U.S. Pat. No. 6,555,485 to Liu etal., U.S. Pat. No. 6,559,007 to Weimer, U.S. Pat. No. 6,610,614 to Niimiet al., and U.S. Pat. No. 6,610,615 to McFadden et al., which areincorporated by reference as if fully set forth herein. A nitrided oxidegate dielectric may also be formed on a wafer using any other methodknown in the art.

In one embodiment, a method for measurement of a specimen is provided.The method includes measuring spectroscopic ellipsometric data of thespecimen. The spectroscopic ellipsometric data may be measured using oneof the optical subsystems described herein. In one embodiment, thespectroscopic ellipsometric data may be measured at wavelengths fromabout 220 nm to about 900 nm. In a different embodiment, thespectroscopic ellipsometric data may be measured at wavelengths fromabout 190 nm to about 300 nm. In such embodiments, the spectroscopicellipsometric data may be measured using one of the non-VUV opticalsubsystems described herein. In one particular example, ellipsometryspectra of the specimen can be acquired using KLA-Tencor's SpectraFx100spectroscopic ellipsometry subsystem at wavelengths from about 220 nm toabout 900 nm, or a combination of spectroscopic ellipsometer and deepultraviolet spectroscopic ellipsometer subsystems at wavelengths fromabout 190 nm to about 300 nm. Alternatively, the spectroscopicellipsometric data may be measured at VUV wavelengths. In this manner,the spectroscopic ellipsometric data may be measured using one of theVUV optical subsystems described herein.

In other embodiments, the spectroscopic ellipsometric data may bemeasured at VUV and non-VUV wavelengths. In such embodiments, thespectroscopic ellipsometric data may be measured using one opticalsubsystem or two different optical subsystems. For example, one opticalsubsystem that is configured to operate at VUV and non-VUV wavelengthsmay be used to measure the spectroscopic ellipsometric data. If the datais measured by two different optical subsystems (one VUV and onenon-VUV), the data may be combined into one data set and then used todetermine a property of the nitrided oxide gate dielectric.

In addition, the method includes determining a nitrogen concentration ofa nitrided oxide gate dielectric formed on the specimen from thespectroscopic ellipsometric data. The nitrogen concentration may bedetermined as described above with respect to atomic concentrationmeasurements. For example, the method may include determining an indexof refraction of the nitrided oxide gate dielectric from thespectroscopic ellipsometric data and determining the nitrogenconcentration from the index of refraction. In a particular example, aquantitative correlation between the measured index of refraction orindex of refraction model (e.g., the BEMA fraction) and the nitrogenconcentration may be determined using measurements of specimens havingknown nitrogen concentration values. The quantitative correlation maythen be used to determine the nitrogen concentration of other specimensbased on a measured index of refraction.

Preferably, measurement of ellipsometry spectra of nitrided oxide gatedielectric films is performed over a broad range of wavelengths suchthat the film thickness and index of refraction can be determined fromthe spectra. Therefore, in some embodiments, the method may also includedetermining a thickness and an index of refraction of the nitrided oxidegate dielectric using the spectroscopic ellipsometric data. In addition,the thickness and index of refraction of a nitrided oxide gatedielectric may be measured simultaneously, which may be particularlyadvantageous due to the strong correlation effect between the thicknessand index of refraction measurements of thin films. In contrast, in thepast, as a general practice, only the thickness could be measured forfilms thinner than 100 angstroms.

In one embodiment, the method may also include measuring reflectometricdata of the specimen. The reflectometric data may be measured at asingle wavelength or at multiple wavelengths (e.g., spectroscopicreflectometric data). The reflectometric data may be measured using oneof the optical subsystem embodiments described herein that is configuredas a reflectometer or spectroscopic reflectometer. In addition, thespectroscopic ellipsometric data and the reflectometric data may bemeasured with one system (e.g., different optical subsystems arrangedwithin a single system). The reflectometric data may be measured atnon-VUV wavelength(s), VUV wavelength(s) or non-VUV and VUV wavelengths.

In some embodiments, the nitrogen concentration of the nitrided oxidegate dielectric may be determined from the spectroscopic ellipsometricdata in combination with the reflectometric data. For example, thereflectometric data may be used to determine a thickness of a nitridedoxide. The thickness and the spectroscopic ellipsometric data may beused in combination to determine an index of refraction of the nitridedoxide, and the index of refraction may be used to determine the nitrogenconcentration.

In some embodiments, measuring the spectroscopic ellipsometric data mayinclude measuring the data at multiple locations on the specimen. Insuch embodiments, the method may also include determining the nitrogenconcentration at the multiple locations and determining a within wafer(WIW) uniformity of the nitrogen concentration. In other words, thenitrogen concentration versus location on the specimen may bedetermined.

In additional embodiments, the method may include removing contaminants(e.g., air-borne molecular contamination (AMC)) from a localized area onthe specimen using a laser-based cleaning subsystem prior tomeasurements of the specimen. The laser-based cleaning subsystem may beconfigured as described further herein. In addition, the laser-basedcleaning subsystem may be arranged in one single system that alsoincludes the optical subsystem that is used for measuring spectroscopicellipsometric data of the specimen. Such configurations of a system aredescribed further herein. Removing contaminants from a specimen justprior to the measurements may substantially increase the accuracy of themeasurements particularly for substantially thin nitrided oxide gatedielectrics.

Measuring the spectroscopic ellipsometric data of the specimen may beperformed during a semiconductor fabrication process. In other words,the spectroscopic ellipsometric measurements may be performed in-situ.In one embodiment, the spectroscopic ellipsometric data may be measuredafter or between individual processes involved in the semiconductorfabrication process (e.g., after formation of the nitrided oxide gatedielectric). Alternatively, the spectroscopic ellipsometric data may bemeasured during an individual process involved in the semiconductorfabrication process (e.g., during formation of the nitrided oxide gatedielectric).

In such embodiments, one of the optical subsystems described herein oranother spectroscopic ellipsometer subsystem, and optionally areflectometer subsystem, may be coupled to a semiconductor fabricationprocess tool. For example, for spectroscopic ellipsometric measurementsof a nitrided oxide gate dielectric, a spectroscopic ellipsometeroptical subsystem may be coupled to a plasma nitridation process tool.However, any of the optical subsystems or systems described herein maybe coupled to any other semiconductor fabrication process tool known inthe art (e.g., a lithography tool, a deposition tool, an anneal tool, anetch tool, a cleaning tool, a chemical-mechanical polishing tool, aplating tool, an ion implantation tool, etc.). Examples of how anoptical subsystem or a measurement system may be coupled to asemiconductor fabrication process tool are described in U.S. Pat. No.6,633,831 to Nikoonahad et al., which is incorporated by reference as iffully set forth herein.

The method may also include monitoring one or more parameters of asemiconductor fabrication process using the nitrogen concentration. Forexample, the nitrogen concentration may be measured repeatedly over timeon many different specimens and the nitrogen concentration values andvariations may be recorded. The data may then be used to determinechanges in parameter(s) of the process. In particular, the nitridationprocess (or individual processes involved in the nitridation process)may be monitored using the nitrogen concentration. Parameter(s) of asemiconductor fabrication process may also, or alternatively, bemonitored using other properties of the nitrided oxide gate dielectricthat are determined as described herein (e.g., the WIW uniformity,thickness, and/or index of refraction of the nitrided oxide gatedielectric). For example, monitoring the parameter(s) of a semiconductorfabrication process may be performed using the index of refraction (n)of the nitrided oxide gate dielectric at a given wavelength (e.g., at193 nm, at 248 nm, at 633 nm) or an index of refraction model parameter(e.g., the BEMA fraction in a Bruggermann Effective Media Approximationmodel) for process control instead of the nitrogen concentration.

In addition, the method may include altering one or more parameters of asemiconductor fabrication process based on the nitrogen concentration.In some embodiments, the parameter(s) may also, or alternatively, bealtered based on the WIW uniformity, thickness, and/or index ofrefraction of the nitrided oxide gate dielectric. Preferably, theparameter(s) that are altered are parameter(s) of a nitridation processor individual processes involved in forming the nitrided oxide gatedielectric (e.g., a thermal oxidation process, a nitridation process, ananneal process, etc.). The parameter(s) may be altered using a feedbacktechnique (e.g., if the nitrogen concentration is measured after thenitridation process) and/or using an in situ control technique (e.g., ifthe nitrogen concentration is measured during the nitridation process).

The parameter(s) are preferably altered to make differences between themeasured nitrogen concentration and a target nitrogen concentrationsmaller. Since the nitrogen concentration can be measured during thesemiconductor fabrication process, the parameter(s) of the process canbe altered in response to the nitrogen concentration much more quicklythan if the other measurement techniques described above (e.g., XPS andSIMS) were used to measure the nitrogen concentration. Therefore, themethods described herein may provide more accurate process control andmay increase yield and throughput of the semiconductor manufacturingprocess. In addition, the parameter(s) that are altered may beparameter(s) of one or more processes performed on the specimen uponwhich the measured nitrided oxide is formed. The parameter(s) may bealtered, in this embodiment, using a feedforward control technique.

Altering the parameter(s) of the semiconductor manufacturing process maybe performed manually (e.g., by an operator) or automatically by aprocessor. The processor may be configured as described herein. Theprocessor may also be coupled to a semiconductor manufacturing processtool. In this manner, the processor may send the changes for theparameter(s) to the semiconductor manufacturing process tool. Aprocessor of the semiconductor manufacturing tool may then make theappropriate changes to the process(es).

The embodiments of the methods described herein for measurement ordetermination of nitrogen concentration have many advantages over thecurrently used methods. For example, the above described methods arerelatively fast (e.g., each measurement takes about 5 seconds to about15 seconds to report results). The above described methods also haverelatively high accuracy (e.g., error for thickness measurements of lessthan about 1.0 angstrom and error for nitrogen concentration of lessthan about 2 at %). Furthermore, the methods for nitrogen concentrationmeasurement or determination described herein have better repeatability(e.g., for 3 sigma, 0.15 angstroms for thickness and 0.5 at % fornitrogen concentration) than currently used methods (e.g., for XPS and 3sigma, 1 angstrom for thickness and 1.0% for nitrogen concentration).

In addition, the above described methods are non-destructive. Therefore,the nitrogen concentration of product wafers may be measured therebyproviding substantially more accurate process monitoring and controlsince measurements do not have to be performed on monitor wafers thatare different than actual product wafers. Moreover, the methods fornitrogen concentration measurement described herein can be used forhigh-volume semiconductor device fabrication using commerciallyavailable optical subsystems such as KLA-Tencor's F5x and SpectraFx100metrology tools. In particular, the methods may be applied to 90 nm, 65nm, and possibly 45 nm generation logic and DRAM devices. Therefore, themethods provide solutions for fast, accurate, stable, and cost effectivemethods to monitor and control the nitrided gate process in developmentand volume manufacturing.

Additional embodiments relate to a computer-implemented method foranalysis of a specimen. The method includes determining a nitrogenconcentration of a nitrided oxide gate dielectric formed on the specimenfrom spectroscopic ellipsometric data generated by measurement of thespecimen. The spectroscopic ellipsometric data may be measured asdescribed above. For example, the spectroscopic ellipsometric data maybe measured at wavelengths from about 220 nm to about 900 nm.Alternatively, the spectroscopic ellipsometric data may be measured atwavelengths from about 190 nm to about 300 nm. In another alternative,the spectroscopic ellipsometric data may be measured at VUV wavelengths.In an additional embodiment, the spectroscopic ellipsometric data may bemeasured at multiple locations on the specimen. The method may alsoinclude determining the nitrogen concentration at the multiple locationsand determining a WIW uniformity of the nitrogen concentration.

In some embodiments, the method may include determining an index ofrefraction of the nitrided oxide gate dielectric from the spectroscopicellipsometric data and determining the nitrogen concentration from theindex of refraction as described above. In another embodiment, themethod may include determining a thickness and an index of refraction ofthe nitrided oxide gate dielectric using the spectroscopic ellipsometricdata. In other embodiments, determining the nitrogen concentration mayinclude determining the nitrogen concentration from the spectroscopicellipsometric data in combination with reflectometric data generated bymeasurement of the specimen.

In additional embodiment, the method may include determining one or moreparameters of a semiconductor fabrication process based on the nitrogenconcentration and/or other determined properties of the nitrided oxidegate dielectric. The method may also include altering the one or moreparameters of the semiconductor fabrication process based on thedetermined parameter(s). The parameter(s) may be altered as describedabove. In further embodiments, the method may include monitoring one ormore parameters of a semiconductor fabrication process using thenitrogen concentration and/or other determined properties of thenitrided oxide gate dielectric. The parameter(s) of the semiconductorfabrication process may be monitored as described above.

Program instructions implementing methods such as those described hereinmay be transmitted over or stored on a carrier medium. The carriermedium may be a transmission medium such as a wire, cable, or wirelesstransmission link, or a signal traveling along such a wire, cable, orlink. The carrier medium may also be a storage medium such as aread-only memory, a random access memory, a magnetic or optical disk, ora magnetic tape.

In an embodiment, a processor may be configured to execute the programinstructions to perform a computer-implemented method according to theabove embodiments. The processor may take various forms, including apersonal computer system, mainframe computer system, workstation,network appliance, Internet appliance, personal digital assistant(“PDA”), television system or other device. In general, the term“computer system” may be broadly defined to encompass any device havinga processor, which executes instructions from a memory medium.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using ActiveX controls, C++ objects,JavaBeans, Microsoft Foundation Classes (“MFC”), or other technologiesor methodologies, as desired.

FIG. 13 illustrates another embodiment of an optical subsystem that maybe included in a system for measurement of a specimen. In thisembodiment, the optical subsystem is configured as a dual channeloptical subsystem such as a dual beam spectrophotometer. This opticalsubsystem may be included in the systems described herein as either afirst optical subsystem or a second optical subsystem. Thisconfiguration is described in more detail in U.S. Pat. No. 5,486,701 toNorton et al. and U.S. Pat. No. 5,608,526 to Piwonka-Corle et al., whichare incorporated by reference as if fully set forth herein. For the sakeof clarity, some of the components and details of the system have beenomitted from FIG. 13 and the corresponding description presented herein.However, it is to be understood that the system illustrated in FIG. 13may be further configured as described in this patent.

The optical subsystem may include lamp 110 (typically a Xenon arc lamp)which emits radiation beam 112 (including visible and/or UV radiation),lamp housing window 114, off-axis paraboloid mirror 116, flip-in UVcutoff filter 118, color filter wheel 120, flat mirror 122, concavemirror 124, aperture mirror 128 with flip-in forty-micron fine focusaperture 130, large achromat 132, field illumination shutter 131, foldmirror 136, and small achromat 138, as described in U.S. Pat. No.5,486,701.

In an alternative embodiment, the optical subsystem may include a lightsource that can generate VUV light. In some embodiments, the opticalsubsystem may include a light source that can generate VUV light as wellas non-VUV light or two light sources, one that can generate VUV lightand another that can generate non-VUV light. Examples of suitable lightsources that can generate such light are described further above.Therefore, the dual channel optical subsystem can perform measurementsof the specimen using VUV light, non-VUV light, or a combinationthereof. If the optical subsystem is configured to perform measurementsusing VUV light and non-VUV light, the dual channel optical subsystemmay include one or more filters (not shown) configured to prevent asubstantial amount of the VUV light from reaching the specimen duringthe additional measurements with the non-VUV light. The one or morefilters may be located in a variety of places in the optical path of thedual channel optical subsystem between light source 110 and objective140. The one or more filters may be further configured as describedabove.

The components described above provide combined beam 142 includingmeasurement beam 125 and field illumination beam 134. Off-axisparaboloid mirror 116 collimates beam 112, which can be optionallyfiltered by flip-in UV cutoff filter 118 and color filter wheel 120.Flip-in UV cutoff filter 118 is used in part to limit the spectrum ofbeam 112 so that when beam 112 is dispersed by a diffraction grating,the first and second order diffraction beams do not overlap. Part ofbeam 112 is reflected by flat mirror 122 onto concave mirror 124 to formmeasurement beam 125. Mirror 124 focuses an image of the arc ontoaperture mirror 128. The radiation emanating from each point in theimage of the arc expands in a uniform cone, typically producing auniform circle of illumination at beam divider 145.

Another part of beam 112, field illumination beam 134, is focused bylarge achromat 132 near fold mirror 136, causing fold mirror 136 toreflect an image of lamp 110 toward small achromat 138. Small achromat138 collects the radiation in beam 134 before it reflects from aperturemirror 128. The aperture is placed at one conjugate of objective 140.The field illumination can be turned off by placing field illuminationshutter 131 in the optical path of field illumination beam 134.

Narrow measurement beam 125 and wide field illumination beam 134 arerejoined at aperture mirror 128, with field illumination beam 134reflecting off the front of aperture mirror 128, and measurement beam125 passing through the aperture. The optical subsystem includesobjective 140, beamsplitter mirror 145, sample beam 146, reference beam148, concave mirror 150, flat mirror 143, reference plate 152 with areference spectrometer pinhole therethrough, sample plate 154 with asample spectrometer pinhole therethrough, second fold mirror 168,diffraction grating 170, sample linear photodiode array 172, referencelinear photodiode array 174, reference photodiode 195, and samplephotodiode 193. Objective 140, which can be a reflective objective (asshown in FIG. 13) or a transmissive objective (not shown) preferably hasseveral selectable magnifications. Therefore, the optical subsystem mayinclude reflective focusing optics or transmissive focusing optics.

The measurement of the relative reflectance spectrum of a specimen,which in this case is shown as wafer 103, will now be described. Whenfield illumination shutter 131 is placed in the path of fieldillumination beam 134, combined beam 142 includes only measurement beam125. Combined beam 142 is split by beam splitter mirror 145, a totallyreflecting mirror placed so as to deflect half of combined beam 142towards objective 140, thus forming sample beam 146, the undeflectedhalf of combined beam 142 forming reference beam 148. Importantly,because sample beam 146 and reference beam 148 are derived from the samesource (lamp 110) and because combined beam 142 is radially uniform,reference beam 148 and sample beam 146 have proportionally dependentspectral intensities. Furthermore, since beamsplitter mirror 145 is atotally reflecting mirror in half of the optical path rather than apartially reflecting mirror in the entire optical path, a continuousbroadband spectrum is reflected with good brightness.

Reference beam 148 does not initially interact with beamsplitter mirror145, but instead illuminates concave mirror 150. Concave mirror 150 isslightly off-axis, so reference beam 148 is reflected onto the reverseface of beamsplitter mirror 145, and flat mirror 143 re-reflectsreference beam 148 into alignment with the reference spectrometerpinhole through plate 152. Flat mirror 143 is provided to realignreference beam 148 with sample beam 146 so that both beams pass thoughtheir respective spectrometer pinholes substantially parallel.

The focal length of concave mirror 150 is such that reference beam 148is in focus at the reference spectrometer pinhole (which extends throughplate 152). The radiation passing through the reference spectrometerpinhole and reflecting from fold mirror 168 is dispersed by diffractiongrating 170. The resulting first order diffraction beam is collected byreference linear photodiode array 174, thereby measuring a referencereflectance spectrum.

Sample beam 146 is reflected from beamsplitter mirror 145 towardsobjective 140, which focuses sample beam 146 onto wafer 103, and thereflected sample beam 146 is focused by objective 140 onto the samplespectrometer pinhole (which extends through plate 154). As describedabove, objective 140 may be a transmissive objective or a reflectiveobjective. Therefore, the optical subsystem may include eitherreflective collecting optics or transmissive collecting optics. However,since objective 140 both focuses the light on the specimen and collectsthe light returned from the specimen, the focusing and collecting opticsare either transmissive or reflective. However, it is to be understoodthat the optical subsystem shown in FIG. 13 may alternatively includereflective focusing optics and transmissive collecting optics or viceversa. For example, the optical subsystem may be configured to useseparate optics for focusing light on the specimen and for collectinglight from the specimen. In this manner, the optical subsystem mayinclude reflective focusing optics and transmissive collecting optics.Alternatively, the optical subsystem may include transmissive focusingoptics and reflective collecting optics.

The reflected sample beam 146 does not interact with beamsplitter mirror145 on the reflected path, because sample beam 146 passes through thespace behind beamsplitter mirror 145, through which reference beam 148also passes. The radiation passing through the sample spectrometerpinhole and reflecting from fold mirror 168 is dispersed by diffractiongrating 170. As with the reference beam, the resulting first orderdiffraction beam of the sample beam is collected by sample linearphotodiode array 172, thereby measuring the sample spectrum.

The relative reflectance spectrum can be simply obtained by processingthe outputs of arrays 172 and 174 in processor 200, by dividing thesample light intensity at each wavelength (the output of array 172) bythe reference intensity at each wavelength (the output of array 174).

In some embodiments, diffraction grating 170 is a concave holographicgrating and the spectrometer pinholes (through plates 152 and 154) are15 mm apart. This embodiment of diffraction grating 170 isholographically corrected to image multiple spectra, since the 15 mmspacing does not allow for both beams to be centered on the grating. Itis also desirable that grating 170 be designed so that the angle ofdetectors 172 and 174 causes reflections from the detectors to propagateaway from the grating.

In an operating mode for measuring the thickness of very thin film 103 a(VTF 103 a) on sample 103, the system employs sample VTF photodiode 193and reference VTF photodiode 195. Dichroic mirror 202 mounted on amoveable arm flips into the beam path immediately beyond aperturedplates 152 and 154. The dichroic mirror reflects UV radiation (withwavelength between 400 nm and 280 nm) and transmits visible light. Thereflected UV from the reference beam is focused by fused silica lens204, reflected by fixed dichroic mirror 206, and finally falls on UVenhanced silicon photodiode 195 (the “reference VTF photodiode”), andthe reflected UV from the sample beam is focused by fused silica lens203, reflected by fixed dichroic mirror 206, and finally falls on UVenhanced silicon photodiode 203 (the “sample VTF photodiode”). Seconddichroic mirror 206 is needed to filter out residual visible light. Theradiation transmitted through first dichroic 202 continues through thenormal spectrometer path.

In a new embodiment, if the dual channel optical subsystem is configuredto perform measurements with VUV light in addition to non-VUV light,dichroic mirror 202 may be configured to reflect VUV radiation andtransmit non-VUV radiation. In such an embodiment, fused lenses 203 and204 may be replaced with calcium fluoride lenses. Such an embodiment maybe further configured as described above.

Each of photodiodes 193 and 195 measures a single intensity value, buttypically this value is an average over a broadband frequency range ofinterest (in the UV range) so that the two photodiodes providesufficient information for calculating a relative reflectance (orreflectance) value representing an average over such broadband frequencyrange. Photodiodes 193 and 195 are preferably selected to havesensitivity to a broad range of wavelengths in the UV band, with bothphotodiodes having substantially similar peak sensitivity wavelengths.When the response of sample photodiode 193 is divided by the response ofreference photodiode 195, the result is a value indicative of therelative reflectance of wafer 103 over wavelengths in the UV band, withthe peak sensitivity wavelength having more weight in the measure ofrelative reflectance than other wavelengths. The measured relativereflectance value can be calibrated to generate a signal indicative ofthe true reflectance of the sample in the UV band.

To avoid the need to apply a complicated algorithm (assuming a weightedaverage of many incident wavelengths) to compute film thickness,processor 200 of the system determines a single effective wavelength forthe broadband UV incident on each of photodiodes 193 and 195. The analogoutput of photodiodes 193 and 195 is digitized (and otherwise processed)in electronic circuitry 190 before undergoing digital processing inprocessor 200.

The optical subsystem shown in FIG. 13 may be further configured asdescribed herein. For example, the dual channel optical subsystem mayinclude one or more hollow optical fibers (not shown). The hollowoptical fiber(s) may be configured as described above. In oneembodiment, the dual channel optical subsystem may include a hollowoptical fiber that transmits light from light source 110 to aperturemirror 128 or another optical component of the dual channel opticalsubsystem. The hollow optical fiber may also be configured such that aposition of the light source may be substantially stationary duringmovement of other optical components of the dual channel opticalsubsystem, as described with respect to FIGS. 10 and 11. The system maybe configured to move these other optical components of the dual channeloptical subsystem with respect to the specimen to perform measurementsat different locations on the specimen. Eliminating movement of thelight source during scanning has advantages that are further describedabove.

In order to use this dual-beam spectrophotometer at VUV wavelengths(either on its own or in combination with other measurement systems), itis advantageous to enclose the optics in an inert atmosphere (exceptpossibly the detectors and their associated optics) and use twodifferential apertures. The optical subsystem shown in FIG. 13 ispreferably disposed within purged environment 208. The purgedenvironment may be provided as described above. For instance, the purgedenvironment may be created within housing 210 that surrounds a least aportion of the optical subsystem. In addition, a system that includesthe optical subsystem shown in FIG. 13 may include a purging subsystem(not shown in FIG. 13) that is configured to maintain a purgedenvironment around the dual channel optical subsystem duringmeasurements by the optical subsystem. The purging subsystem may be adifferential purging subsystem as described above. Housing 210 may alsobe configured as described above.

One or more components of the optical subsystem may be disposed outsideof the housing in a non-purged environment. For example, as shown inFIG. 13, concave mirror 150 may be located outside of the housing in anon-purged environment. In addition, although detectors 172 and 174 areshown to be located inside of the housing in the purged environment, thedetectors may alternatively be located outside of the housing in anon-purged environment. In addition, other components coupled to theoptical subsystem are located outside of the housing and in a non-purgedenvironment. These components include electronic circuitry 190 andprocessor 200. Placing components such as the detectors, electroniccircuitry, and processor outside of the purged environment may reducethe area of the purged environment within the housing thereby reducingthe cost and time of creating the purged environment.

Concave mirror 150 may be placed outside of the purged environment, notfor space considerations, but to create optical conditions in thereference channel that are similar to those in the sample channel. Forexample, the dual-beam spectrophotometer has two light paths (sample andreference) which run parallel for most of their lengths. However, thereference channel cannot be reflected from the wafer. Unless a seconddifferential aperture is added near reference mirror 150, the referencebeam will not experience the same amount of absorption as the samplebeam, which passes through a differential aperture within, or near, theobjective. For example, housing 210 includes differential aperture 212through which sample beam 146 is focused onto specimen 103 by objective140. The differential aperture may be configured as described above.Outside of the differential aperture, sample beam 146 passes through anon-purged space. Therefore, the sample beam may be partially absorbedby unwanted molecules in this non-purged space. This absorption willalter the characteristics of the light that is measured by sample linearphotodiode array 172.

If reference beam 148 is not similarly altered to account for absorptionof the sample beam, the measurements of the dual beam spectrophotometermay be inaccurate. Therefore, the optical subsystem is configured suchthat at least a portion of the reference beam passes through anon-purged space. One embodiment shown in FIG. 13 includes positioningconcave mirror 150 outside of housing 210. Housing 210 includes seconddifferential aperture 214 through which reference beam 148 is directedonto concave mirror 150. Differential aperture 214 may be configured asdescribed above.

Ideally, the reference aperture is at a substantially similar distancefrom the reference mirror surface as the objective aperture is from thewafer surface. For example, the concave mirror is spaced fromdifferential aperture 214 by a distance that is approximately equal tothe distance that specimen 103 is spaced from differential aperture 212.In this manner, the reference beam may pass through an amount ofnon-purged space that is approximately equal to the amount of non-purgedspace through which the sample beam passes. As such, the absorption ofreference beam 148 and sample beam 146 may be approximately equal due tounwanted molecules in both non-purged spaces. In this manner, thepurging subsystem is configured to maintain the same level of purging inboth channels of the dual channel optical subsystem. Consequently, thelight that is measured by sample linear photodiode array 172 andreference linear photodiode array 174 may be altered in substantiallyequal ways thereby increasing the accuracy of the measurements of thedual beam spectrophotometer.

Such “normalization” of the optical channels for absorption may beimplemented if other components of the optical subsystem are disposed inthe non-purged environment. For example, if detectors 172 and 174 arelocated outside of the purged environment, two separate differentialapertures (not shown) may be formed in the housing to allow thereference and sample beams to by directed to the different detectors ina spatially separated manner. The space between the photosensitiveelements of the detectors and the respective differential apertures maybe approximately equal such that absorption of the reference and samplebeams outside of the housing may be approximately equal.

In addition, many different optical subsystems known in the art havemultiple channels such as a beam profile reflectometer and some opticalsubsystems designed particularly for inspection. Multiple channels ofany such optical subsystem, which is disposed within a purgedenvironment, may be “normalized” for absorption in a similar manner.

A system configured for measurement of a specimen that includes a dualchannel optical subsystem may be further configured as described herein.For example, the dual channel optical subsystem may be included in asystem that includes one or more additional optical subsystems. The oneor more additional subsystems may be configured to perform additionalmeasurements of the specimen using non-VUV light. The one or moreadditional optical subsystems may be disposed within a non-purgedenvironment. For example, the dual channel optical subsystem may beincluded in the system illustrated in FIG. 1 as the first opticalsubsystem in place of the ellipsometer. Such a system may include asecond optical subsystem that is configured as a reflectometer asdescribed with respect to FIG. 1. Alternatively, the reflectometer maybe replaced with a variety of other optical subsystems such as anellipsometer (e.g., a single wavelength ellipsometer or a spectroscopicellipsometer), a scatterometer, etc.

In some embodiments, a system may include two or more optical subsystemsdisposed within a purged environment. For example, the system mayinclude a dual channel optical subsystem such as that shown in FIG. 13in addition to another optical subsystem, both disposed within a purgedenvironment. One such embodiment is illustrated in FIG. 14. Thereference numerals for components of the dual channel optical subsystemare not included in FIG. 14. However, all of the components of the dualchannel optical subsystem shown in FIG. 14 are also illustrated in FIG.13 along with corresponding reference numerals. Therefore, for detailsof the components of the dual channel optical subsystem illustrated inFIG. 14, one can refer to FIG. 13 and the corresponding description.

As shown in FIG. 14, the additional optical subsystem disposed withinpurged environment 208 along with the dual channel optical subsystem hasthe configuration of the first optical subsystem illustrated in FIG. 1.In particular, the additional optical subsystem in this embodiment isconfigured as an ellipsometer. The ellipsometer includes light source74, polarizer 76, analyzer 78, and detector 82, all of which may beconfigured as described above. In addition, the ellipsometer mayoptionally include one or more filters 84 and spectrometer 80, both ofwhich may also be configured as described above. In one embodiment, theadditional optical subsystem may be configured as a single wavelengthellipsometer. The single wavelength ellipsometer may be configured toperform measurements of the specimen at a wavelength of 157 nm. In analternative embodiment, the additional optical subsystem may beconfigured as a spectroscopic ellipsometer. In such an embodiment, lightsource 74 may be a VUV light flash lamp.

In other embodiments, the additional optical subsystem disposed in thesame purged environment as the dual channel optical subsystem may haveother configurations. For example, the additional optical subsystem maybe configured as a reflectometer, a spectroscopic reflectometer, ascatterometer, a spectroscopic scatterometer, a different dual channeloptical subsystem, or any other measurement subsystem known in the art.In further embodiments, more than one additional optical subsystem maybe disposed in the same purged environment as the dual channel opticalsubsystem. For example, additional optical subsystems configured as anellipsometer and a reflectometer may be disposed within the same purgedenvironment as the dual channel optical subsystem. Alternatively,additional optical subsystems configured as an ellipsometer and ascatterometer may be disposed within the same purged environment as thedual channel optical subsystem.

As shown in FIG. 14, housing 210 has two differential apertures 212 and214 for the dual channel optical subsystem. The sample beam directed tothe specimen passes through differential aperture 212, and the referencebeam directed to mirror 150 passes through differential aperture 214. Asdescribed further above, the two differential apertures and the purgingsubsystem (not shown in FIG. 14) are configured such that the same levelof purging is maintained in both channels of the dual channel opticalsubsystem. In addition, housing 210 includes differential aperture 18for the additional optical subsystem. The light directed to specimen 10from the additional optical subsystem and returned from specimen 10 tothe additional optical subsystem passes through this differentialaperture. Therefore, the housing contains three differential apertures.In addition, if more than two optical subsystems are disposed within thepurged environment within housing 210, the housing may have additionaldifferential apertures for these additional optical subsystems.

FIG. 15 illustrates one embodiment of two optical subsystems that areconfigured to perform measurements of a specimen using VUV light.Although only two optical subsystems are illustrated in FIG. 15, it isto be understood that more than two optical subsystems may be includedin such an embodiment. The two optical subsystems shown in FIG. 15 maybe included in a system configured for measurement of a specimen. Such asystem may be further configured as described herein.

The two optical subsystems are coupled to purging subsystem 216. Purgingsubsystem 216 is configured to maintain purged environment 217 aroundthe two optical subsystems during measurements of specimen 10. Thepurging subsystem may be further configured as described above. Forexample, the purging subsystem may be a differential purging subsystem.In addition, the purging subsystem is configured to maintainapproximately the same level of purging in the two optical subsystems.For example, the two optical subsystems may be disposed within housing218. The housing may be configured as described above. In addition, thehousing includes differential aperture 220, which is configured suchthat VUV light from the two optical subsystems can be directed tospecimen 10 through the differential aperture and such that lightreturned from the specimen can pass through the differential apertureand be collected by the two optical subsystems. Differential aperture220 may be further configured as described above.

One of the two optical subsystems in this embodiment is configured as anellipsometer. The ellipsometer includes light source 74, polarizer 76,analyzer 78, and detector 82, all of which may be configured asdescribed above. In addition, the ellipsometer may optionally includeone or more filters 84 and spectrometer 80, both of which may also beconfigured as described above. In one embodiment, the optical subsystemmay be configured as a single wavelength ellipsometer. The singlewavelength ellipsometer may be configured to perform measurements of thespecimen at a wavelength of 157 nm. In an alternative embodiment, theoptical subsystem may be configured as a spectroscopic ellipsometer. Insuch an embodiment, light source 74 may be a VUV light flash lamp.

The other of the two optical subsystems in this embodiment is configuredas a reflectometer, which may be configured as described above. Forexample, the second optical subsystem includes light source 98. Lightsource 98 may be a monochromatic or near monochromatic light source. Inthis embodiment, the reflectometer may be configured as a singlewavelength reflectometer. In another embodiment, the light source may bea polychromatic light source or broadband light source. In such anembodiment, the reflectometer may be configured as a spectroscopicreflectometer. The second optical subsystem also includes beam splitter100, lens 102, diffraction grating 104, and detector array 106, all ofwhich may be configured as described above.

In some embodiments, therefore, the two optical subsystems shown in FIG.15 may include an ellipsometer (e.g., a single wavelength ellipsometeror a broadband ellipsometer) and a reflectometer (e.g., a singlewavelength reflectometer or a broadband spectroscopic reflectometer). Inanother embodiment, the two optical subsystems may include a broadbandspectroscopic ellipsometer as described above and a broadband reflectivespectrometer, which is described in more detail below.

As described further above, the optical subsystem that is configured asan ellipsometer may include one or more filters 84 if the ellipsometeris configured to perform measurements of the specimen using VUV lightand non-VUV light. The one or more filters may be inserted in theoptical path of the ellipsometer to prevent a substantial amount of theVUV light from reaching the specimen during measurements with non-VUVlight. In this manner, the specimen may be protected from potentialdamage by prolonged exposure to the VUV light. In a similar manner, theoptical subsystem that is configured as a reflectometer may also includeone or more filters 222 if the reflectometer is configured to performmeasurements of the specimen using VUV light and non-VUV light. One ormore filters 222 may not be placed in the optical path of thereflectometer during measurements with VUV light, but may be inserted inthe optical path of the reflectometer during measurements with non-VUVlight. One or more filters 222 may be configured to prevent asubstantial amount of the VUV light from reaching the specimen duringmeasurements with the non-VUV light thereby preventing potential damageof the specimen by prolonged and unnecessary exposure to the VUV light.

The two optical subsystems are shown in FIG. 15 to direct and collectlight through the same differential aperture. In this manner, the twooptical subsystems may be configured to measure approximately the samemeasurement spot on the specimen either simultaneously or sequentially.Alternatively, the two optical subsystems may be configured to directand collect light through different differential apertures (not shown).For example, the optical subsystems may be laterally spaced apart withinhousing 218, and each optical subsystem may be coupled to a differentdifferential aperture. As such, the two optical subsystems may beconfigured to measure different measurement spots on the specimensimultaneously. In addition, the two optical subsystems may measure thesame measurement spot on the specimen sequentially.

A system that includes two optical subsystems configured to measure aspecimen using VUV light such as those shown in FIG. 15 may also includeone or more additional optical subsystems (not shown). The additionaloptical subsystem(s) may be configured to perform additionalmeasurements of the specimen using non-VUV light. Therefore, theadditional optical subsystem(s) may be disposed within a non-purgedenvironment. In some embodiments, a first portion of the additionaloptical subsystem(s) may be disposed within the purged environment, anda second portion of the additional optical subsystem(s) may be disposedwithin a non-purged environment. One such embodiment is describedfurther below. In addition, the two optical subsystems and theadditional optical subsystem(s) may have at least one common opticalcomponent. The common optical component(s) may include the first portionof the additional optical subsystem(s) that is disposed within thepurged environment. An example of such an embodiment is describedfurther below. In some embodiments, the additional optical subsystem(s)may include a beam profile ellipsometer, a beam profile reflectometer, abroadband reflective spectrometer, or a combination thereof. Suchoptical subsystems are described further below.

FIG. 16 illustrates another embodiment of two optical subsystems thatare configured to perform measurements of a specimen using VUV light.The optical subsystems shown in FIG. 16 are described in more detail inU.S. Pat. No. 6,515,746 to Opsal et al., which is incorporated byreference as if fully set forth herein. Some of the non-essentialdetails of the system presented in this patent have been omitted fromthe description corresponding to FIG. 16 presented herein. However, itis to be understood that the system illustrated in FIG. 16 may befurther configured as described in this patent. In addition, it will beobvious upon reading the description of several embodiments providedherein that the system illustrated in FIG. 16 has been altered todramatically improve upon the system described in U.S. Pat. No.6,515,746 to Opsal et al. The alterations include, for example, alteringthe system to include VUV optical subsystems and disposing the VUVoptical subsystems in a purged environment. Other changes will beevident upon reading the description of FIG. 16 provided herein.

Although only two VUV optical subsystems are illustrated in FIG. 16, itis to be understood that more than two VUV optical subsystems may beincluded in such an embodiment. The two VUV optical subsystems shown inFIG. 16 may be included in a system configured for measurement of aspecimen. Such a system may be further configured as described herein.

One of the two VUV optical subsystems is configured as a broadbandreflective spectrometer. The other VUV optical subsystem is configuredas a broadband spectroscopic ellipsometer. Both VUV optical subsystemsare disposed within purged environment 224. Purged environment 224 maybe provided and maintained by purging subsystem 226, which may beconfigured as described above. For example, the purging subsystem may becoupled to housing 228, and in one embodiment may be a differentialpurging subsystem. The housing may be configured as described above. Inaddition, the purging subsystem is configured to maintain approximatelythe same level of purging in both of the two VUV optical subsystems.

Broadband reflective spectrometer (BRS) 230 simultaneously probesspecimen 10 with multiple wavelengths of VUV light. BRS 230 uses lens232 and includes a broadband spectrometer 234 which can be of any typecommonly known and used in the prior art. Lens 232 may be a transmissiveoptical component formed of a material such as calcium fluoride (CaF₂).Such a lens may be a spherical, microscope objective lens with a highnumerical aperture (on the order of 0.90 NA) to create a large spread ofangles of incidence with respect to the sample surface, and to create aspot size of about one micron in diameter. Alternatively, lens 232 maybe a reflective optical component. Such a lens may have a lowernumerical aperture (on the order of 0.4 NA) and may be capable offocusing light to a spot size of about 10–15 microns. Spectrometer 234shown in FIG. 16 includes lens 236, aperture 238, dispersive element240, and detector array 242. Lens 236 may be formed of CaF₂.

During operation, probe beam 244 from VUV light source 246 is collimatedby lens 245, directed by mirror 243, and focused through differentialaperture 233 and onto specimen 10 by lens 232. The VUV light source mayinclude any of the light sources described above. Lens 245 may be formedof CaF₂. The differential aperture may be configured as described above.In some embodiments, BRS 230 may be configured to perform additionalmeasurements of the specimen using non-VUV light. The non-VUV light maybe provided by VUV light source 246. Alternatively, the non-VUV lightmay be provided by light source 283 or another light source (not shown).In such an embodiment, BRS may include one or more filters 231.Filter(s) 231 may be configured to prevent a substantial amount of theVUV light from reaching the specimen during the measurements with thenon-VUV light. Therefore, the filter(s) may protect the specimen fromunwanted exposure to the VUV light during non-VUV measurements. Duringmeasurements with the VUV light, filter(s) 231 may be moved out of theoptical path of beam 244 manually, mechanically, or automatically.Although filter(s) 231 are shown to be located between mirror 248 andlens 232, it is to be understood that the filter(s) may be positioned ata variety of locations in the optical path of beam 244 between VUV lightsource 246 and lens 232.

Light reflected from the surface of the sample passes throughdifferential aperture 233 and lens 232, and is directed by mirror 248(through mirror 250) to spectrometer 234. Lens 236 focuses the probebeam through aperture 238, which defines a spot in the field of view onthe specimen surface to analyze. Dispersive element 240, such as adiffraction grating, prism, or holographic plate, angularly dispersesthe beam as a function of wavelength to individual detector elementscontained in detector array 242.

The different detector elements measure the optical intensities ofdifferent wavelengths of light contained in the probe beam, preferablysimultaneously. Alternately, detector 242 can be a charge-coupled device(“CCD”) camera or a photomultiplier with suitably dispersive orotherwise wavelength selective optics. It should be noted that amonochrometer could be used to measure the different wavelengthsserially (one wavelength at a time) using a single detector element.Further, dispersive element 240 can also be configured to disperse thelight as a function of wavelength in one direction, and as a function ofthe angle of incidence with respect to the specimen surface in anorthogonal direction, so that simultaneous measurements as a function ofboth wavelength and angle of incidence are possible. Processor 252processes the intensity information measured by detector array 242.

Although all optical components of BRS 230 are shown in FIG. 16 to belocated within the housing and in purged environment 224, it is to beunderstood that one or more components of the BRS can be located outsideof the housing in the non-purged environment. For example, VUV lightsource 246 may be located outside of the housing in the non-purgedenvironment. Light from the VUV light source may be directed through adifferential aperture (not shown) formed in the housing to lens 245.This differential aperture may also be configured as described herein.

Broadband spectroscopic ellipsometer (BSE) 254 is also configured toperform measurements of the specimen using VUV light. BSE 254 includespolarizer 256, focusing mirror 258, collimating mirror 260, rotatingcompensator 262, and analyzer 264. In some embodiments, BSE 254 may beconfigured to perform additional measurements of the specimen usingnon-VUV light. The non-VUV light may be provided by VUV light source246. Alternatively, the non-VUV light may be provided by light source283 or another light source (not shown). In such an embodiment, BSE mayinclude one or more filters 235. Filter(s) 235 may be configured toprevent a substantial amount of the VUV light from reaching the specimenduring the measurements with the non-VUV light. Therefore, the filter(s)may protect the specimen from unwanted exposure to the VUV light duringnon-VUV measurements. During measurements with the VUV light, filter(s)235 may be moved out of the optical path of beam 244 manually,mechanically, or automatically. Although filter(s) 235 are shown to belocated between mirror 266 and polarizer 256, it is to be understoodthat the filter(s) may be positioned at a variety of locations in theoptical path of beam 244 between VUV light source 246 and specimen 10.

In operation, mirror 266 directs at least part of probe beam 244 topolarizer 256, which creates a known polarization state for the probebeam, preferably a linear polarization. Mirror 258 focuses the beamthrough differential aperture 233 onto the specimen surface at anoblique angle, ideally on the order of 70 degrees to the normal of thespecimen surface. Based upon well known ellipsometric principles, thereflected beam will generally have a mixed linear and circularpolarization state after interacting with the specimen, based upon thecomposition and thickness of the specimen's film 268 and substrate 270.

The reflected beam passes through differential aperture 233 and iscollimated by mirror 260, which directs the beam to rotating compensator262. Compensator 262 introduces a relative phase delay δ (phaseretardation) between a pair of mutually orthogonal polarized opticalbeam components. Compensator 262 is rotated at an angular velocity cabout an axis substantially parallel to the propagation direction of thebeam, preferably by electric motor 272. Analyzer 264, preferably anotherlinear polarizer, mixes the polarization states incident on it. Bymeasuring the light transmitted by analyzer 264, the polarization stateof the reflected probe beam can be determined.

Mirror 250 directs the beam to spectrometer 234, which simultaneouslymeasures the intensities of the different wavelengths of light in thereflected probe beam that pass through the compensator/analyzercombination. Processor 252 receives the output of detector 242, andprocesses the intensity information measured by detector 242 as afunction of wavelength and as a function of the azimuth (rotational)angle of the compensator 262 about its axis of rotation, to solve theellipsometric values ψ and Δ as described in U.S. Pat. No. 5,877,859 toAspnes et al., which is incorporated by reference as if fully set forthherein.

Although all optical components of BSE 254 are shown in FIG. 16 to belocated within the housing and in purged environment 224, it is to beunderstood that one or more components of the BSE can be located outsideof the housing in the non-purged environment. For example, VUV lightsource 246 may be located outside of the housing in the non-purgedenvironment. Light from the VUV light source may be directed through adifferential aperture (not shown) formed in the housing to lens 245.This differential aperture may also be configured as described herein.

A system that includes the broadband reflective spectrometer andbroadband spectroscopic ellipsometer described above may also includeadditional optical subsystem(s) configured to perform additionalmeasurements of the specimen using non-VUV light. For example, thesystem may include non-VUV optical subsystems configured as a beamprofile ellipsometer, a beam profile reflectometer, another opticalsubsystem, or a combination thereof. The one or more additional opticalsubsystems may be disposed within a non-purged environment. For example,at least a portion of the additional optical subsystem(s) are disposedwithin a non-purged environment outside of housing 228.

Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No. 5,181,080to Fanton et al., which is incorporated by reference as if fully setforth herein. BPE 274 includes laser 283 that generates probe beam 284.Laser 283 may be a solid state laser diode from Toshiba Corp. whichemits a linearly polarized 3 mW beam at 673 nm. BPE 274 also includesquarter wave plate 276, polarizer 278, lens 280, and quad detector 282.In operation, linearly polarized probe beam 284 is focused on specimen10 by lens 232 through differential aperture 233. Light reflected fromthe sample surface passes up through differential aperture 233 and lens232, through mirrors 248, 286, and 288, and is directed into BPE 274 bymirror 290.

The position of the rays within the reflected probe beam correspond tospecific angles of incidence with respect to the specimen's surface.Quarter-wave plate 276 retards the phase of one of the polarizationstates of the beam by 90 degrees. Linear polarizer 278 causes the twopolarization states of the beam to interfere with each other. Formaximum signal, the axis of polarizer 278 should be oriented at an angleof 45 degrees with respect to the fast and slow axis of quarter-waveplate 276. Detector 282 is a quad-cell detector with four radiallydisposed quadrants that each intercept one quarter of the probe beam andgenerate a separate output signal proportional to the power of theportion of the probe beam striking that quadrant.

The output signals from each quadrant are sent to processor 252. Bymonitoring the change in the polarization state of the beam,ellipsometric information, such as ψ and Δ, can be determined. Todetermine this information, processor 252 takes the difference betweenthe sums of the output signals of diametrically opposed quadrants, avalue which varies linearly with film thickness for very thin films.

Beam profile reflectometry (BPR) is discussed in U.S. Pat. No. 4,999,014to Gold et al., which is incorporated by reference as if fully set forthherein. BPR 292 includes laser 283, lens 294, beam splitter 296, and twolinear detector arrays 298 and 300 to measure the reflectance of thesample. In operation, linearly polarized probe beam 284 is focusedthrough differential aperture 233 onto specimen 10 by lens 232, withvarious rays within the beam striking the sample surface at a range ofangles of incidence. Light reflected from the specimen surface passes upthrough differential aperture 233 and lens 232, through mirrors 248 and286, and is directed into BPR 292 by mirror 288. The position of therays within the reflected probe beam correspond to specific angles ofincidence with respect to the specimen's surface. Lens 294 spatiallyspreads the beam two-dimensionally. Beam splitter 296 separates the Sand P components of the beam, and detector arrays 298 and 300 areoriented orthogonal to each other to isolate information about S and Ppolarized light. The higher angles of incidence rays will fall closer tothe opposed ends of the arrays. The output from each element in thediode arrays will correspond to different angles of incidence. Detectorsarrays 298 and 300 measure the intensity across the reflected probe beamas a function of the angle of incidence with respect to the specimensurface. Processor 252 receives the output of detector arrays 298 and300, and derives the thickness and refractive index of thin film layer268 based on these angular dependent intensity measurements by utilizingvarious types of modeling algorithms. Optimization routines which useiterative processes such as least square fitting routines are typicallyemployed.

The system shown in FIG. 16 may also include additional components suchas detector/camera 302. Detector/camera 302 is positioned above mirror290, and can be used to view reflected beams off of specimen 10 foralignment and focus purposes.

In order to calibrate BPE 274, BPR 292, BRS 230, and BSE 254, the systemmay include wavelength stable calibration reference ellipsometer 304used in conjunction with a reference sample (not shown). For calibrationpurposes, the reference sample ideally consists of a thin oxide layerhaving a thickness, d, formed on a silicon substrate. However, ingeneral the sample can be any appropriate substrate of knowncomposition, including a bare silicon wafer, and silicon wafersubstrates having one or more thin films thereon. The thickness d of thelayer need not be known or be consistent between periodic calibrations.

Ellipsometer 304 includes light source 306, polarizer 308, lenses 310and 312, rotating compensator 314, analyzer 316, and detector 318.Compensator 314 is rotated at an angular velocity ψ about an axissubstantially parallel to the propagation direction of beam 320,preferably by electric motor 322. It should be noted that thecompensator can be located either between the specimen and the analyzer(as shown in FIG. 16) or between the specimen and polarizer 308. Itshould also be noted that polarizer 308, lenses 310 and 312, compensator314, and polarizer 316 are all optimized in their construction for thespecific wavelength of light produced by light source 306, whichmaximizes the accuracy of ellipsometer.

Light source 306 produces a quasi-monochromatic probe beam 320 having aknown stable wavelength and stable intensity. This can be donepassively, where light source 306 generates a very stable outputwavelength which does not vary over time (i.e., varies less than 1%).Examples of passively stable light sources are a helium-neon laser, orother gas discharge laser systems. Alternately, a non-passive system canbe used where the light source includes a light generator (not shown)that produces light having a wavelength that is not precisely known orstable over time, and a monochrometer (not shown) that preciselymeasures the wavelength of light produced by the light generator.Examples of such light generators include laser diodes, or polychromaticlight sources used in conjunction with a color filter such as a grating.In either case, the wavelength of beam 320, which is a known constant ormeasured by a monochrometer, is provided to processor 252 so thatellipsometer 304 can accurately calibrate the optical measurementdevices in the system.

Operation of ellipsometer 304 during calibration is further described inU.S. Pat. No. 6,515,746. Briefly, beam 320 enters detector 318, whichmeasures the intensity of the beam passing through thecompensator/analyzer combination. Processor 252 processes the intensityinformation measured by detector 318 to determine the polarization stateof the light after interacting with the analyzer, and therefore theellipsometric parameters of the sample. This information processingincludes measuring beam intensity as a function of the azimuth(rotational) angle of the compensator about its axis of rotation. Thismeasurement of intensity as a function of compensator rotational angleis effectively a measurement of the intensity of beam 320 as a functionof time, since the compensator angular velocity is usually known and aconstant.

By knowing the composition of the reference sample, and by knowing theexact wavelength of light generated by light source 306, the opticalproperties of the reference sample such as film thickness d, refractiveindex and extinction coefficients, etc., can be determined byellipsometer 304. Once the thickness d of the film has been determinedby ellipsometer 304, then the same sample is probed by the other opticalmeasurement devices BPE 274, BPR 292, BRS 230, and BSE 254 which measurevarious optical parameters of the sample. Processor 252 then calibratesthe processing variables used to analyze the results from these opticalmeasurement devices so that they produce accurate results. In the abovedescribed calibration techniques, all system variables affecting phaseand intensity are determined and compensated for using the phase offsetand reflectance normalizing factor discussed in U.S. Pat. No. 6,515,746,thus rendering the optical measurements made by these calibrated opticalmeasurement devices absolute.

The above described calibration techniques are based largely uponcalibration using the derived thickness d of the thin film. However,calibration using ellipsometer 304 can be based upon any of the opticalproperties of the reference sample that are measurable or determinableby ellipsometer 304 and/or are otherwise known, whether the sample has asingle film thereof, has multiple films thereon, or even has no filmthereon (bare sample).

As shown in FIG. 16, a first portion of the non-VUV optical subsystemsmay be disposed within the purged environment, and a second portion ofthe non-VUV optical subsystems may be disposed within a non-purgedenvironment. For example, a first portion of BPE 274 is disposed outsideof housing 228, and a second portion of BPE 274 is disposed within thehousing. The first portion of BPE 274 that is disposed outside ofhousing 228 in the non-purged environment includes laser 283, mirror288, mirror 290, quarter wave plate 276, polarizer 278, lens 280, andquad detector 282. The second portion of BPE 274 that is disposed insideof housing 228 in the purged environment includes mirrors 243, 266, 286,and 248, and lens 232. Housing 228 may include differential apertures(not shown) or relatively small sections of material (not shown) thatare transparent to the non-VUV light such that the light can pass fromcomponents of BPE 274 outside of housing 228 to components inside of thehousing and vice versa.

In addition, a first portion of BPR 292 is disposed outside of housing228, and a second portion of BPR 292 is disposed within the housing. Thefirst portion of BPR 292 that is disposed outside of housing 228 in thenon-purged environment includes laser 283, mirror 288, lens 294, beamsplitter 296, and linear detector arrays 298 and 300. The second portionof BPR 292 that is disposed inside of housing 228 within the purgedenvironment includes mirrors 243, 266, 286, and 248, and lens 232.Housing 228 may include differential apertures (not shown) or relativelysmall sections of material (not shown) that are transparent to thenon-VUV light such that the light can pass from components of BPR 292outside of housing 228 to components inside of the housing and viceversa.

Furthermore, a first portion of ellipsometer 304 is disposed outside ofhousing 228, and a second portion of ellipsometer 304 is disposed withinhousing 228. The first portion of ellipsometer 304 that is disposedoutside of the housing in the non-purged environment includes lightsource 306 and polarizer 308. The second portion of ellipsometer 304that is disposed inside of the housing in the purged environmentincludes lenses 310 and 312, rotating compensator 314, analyzer 316, anddetector 318. Housing 228 may include a differential aperture (notshown) or a relatively small section of material (not shown) that istransparent to the non-VUV light such that the light can pass fromcomponents of ellipsometer 304 outside of housing 228 to componentsinside of the housing.

In some embodiments, the VUV optical subsystems and the non-VUV opticalsubsystems may have at least one common optical component. The commonoptical component(s) generally will include optical component(s)included in the non-VUV optical subsystems that are disposed in thepurged environment. For example, lens 232 is common to BPE 274, BPR 292,BRS 230, and BSE 254. In addition, lens 232 is disposed within housing228 in purged environment 224. In a similar manner, mirrors 243, 266,286, and 248 are common to BPE 274, BPR 292, BRS 230, and BSE 254 andare disposed within housing 228 in purged environment 224. Ellipsometer304, as shown in FIG. 16, does not have any optical components that arecommon to the other optical subsystems. Such separation from the otheroptical subsystems may be appropriate since the ellipsometer is used tocalibrate the other optical subsystems.

In another preferred embodiment, the use of measurement technologyoperating at VUV wavelengths is combined with a desorber to removemolecular contamination from the surface of a specimen prior tomeasurement. Molecules from the environment can adhere to the surface ofthe specimen. Typically, these molecules will interact more stronglywith the short wavelength radiation in the VUV than with longerwavelengths and can cause misleading or inaccurate measurement results.In the preferred embodiment, these molecules are removed immediatelyprior to the measurement by a laser beam as described in U.S. patentapplication Ser. No. 10/056,271 entitled “Laser-Based Cleaning DeviceFor Film Analysis Tool” filed on Jan. 23, 2002, by Janik et al., whichis incorporated by reference as if fully set forth herein. Inalternative embodiments, the specimen is cleaned by a hot plate orradiant heat prior to being loaded in the measurement system. Someexamples of hot plates are illustrated in U.S. Pat. No. 6,261,853 toHowell et al., U.S. Pat. No. 6,519,045 to Kwon, and U.S. Pat. No.6,624,393 to Howell et al., all of which are incorporated by referenceas if fully set forth herein.

All of the embodiments of a system configured for measurement of aspecimen described herein may include a cleaning subsystem. The cleaningsubsystem may be configured to remove contaminants from the specimenprior to measurement. The system also includes an optical subsystemconfigured to perform measurements of the specimen using VUV light. Theoptical subsystem is disposed within a purged environment. In contrast,the cleaning subsystem is disposed within a non-purged environment.

FIG. 17 illustrates one embodiment of a system configured formeasurement of a specimen that includes a cleaning subsystem. Thisembodiment of the system is shown to include an optical subsystem thatin this embodiment is configured as the first optical subsystem of FIG.1, which is described in further detail above. However, the opticalsubsystem may include any optical subsystem described herein or known inthe art. For example, the optical subsystem may be configured as anellipsometer, a reflectometer, a scatterometer, a dual beamspectrophotometer, or a combination thereof. The optical subsystem isdisposed within purged environment 14, which is created and maintainedin housing 16 by a purging subsystem (not shown in FIG. 17). The purgingsubsystem may be configured as described above. For example, the purgingsubsystem may be a differential purging subsystem that is configured toprovide the purged environment for the optical subsystem.

In some embodiments, the optical subsystem may also be configured toperform measurements of specimen 10 using VUV light as well as non-VUVlight. For example, the optical subsystem may include one light sourcethat is configured to provide both VUV light and non-VUV light.Alternatively, the optical subsystem may include two light sources, oneconfigured to provide VUV light, and another configured to providenon-VUV light. Such light sources and configurations are describedfurther above. In embodiments that the optical subsystem is configuredto perform measurements using VUV light and non-VUV light, the opticalsubsystem may include one or more filters 84. The filter(s) may beconfigured to prevent a substantial amount of the VUV light fromreaching the specimen during measurement with the non-UV light. Thefilter(s) may be further configured as described above.

The system may or may not also include an additional optical subsystemconfigured to perform additional measurements of specimen 10 usingnon-VUV light. For example, as shown in FIG. 17, the system may includean optical subsystem that is configured as the second optical subsystemof FIG. 1 which is generally configured as a reflectometer. However, thesystem illustrated in FIG. 17 may include optical subsystems other thanreflectometers that are configured to measure the specimen using non-VUVlight such as ellipsometers, scatterometers, dual beamspectrophotometers, a combination thereof and/or any other non-VUVoptical subsystem known in the art. As shown in FIG. 17, the additionaloptical subsystem may be disposed within a non-purged environmentoutside of housing 16.

In an alternative embodiment, the system may include an additionaloptical subsystem (not shown in FIG. 17) that is configured to performadditional measurements of the specimen using VUV light. Such anadditional optical subsystem may be disposed in purged environment 14.Embodiments that include more than one optical subsystem disposed in thesame purged environment are illustrated in FIGS. 15 and 16, and thesystem illustrated in FIG. 17 may include such optical subsystems. Inaddition, it is to be noted that the number of optical subsystems thatare disposed in one purged environment is not limited to two. In fact,it is conceivable that three, four, five, or any other number of opticalsubsystems may be disposed in the same purged environment. The opticalsubsystems may be further configured as described herein.

The system shown in FIG. 17 also includes cleaning subsystem 324disposed in non-purged environment 326. For example, cleaning subsystem324 is disposed outside of housing 16 and outside of purged environment14 that is created and maintained within housing 16. As shown in FIG.17, the cleaning subsystem and the optical subsystems are coupled to thesame stage (i.e., stage 12). In this manner, the optical subsystem(s)may perform measurements on the specimen while the cleaning subsystem isremoving contaminants from the specimen. Alternatively, the cleaningsubsystem may be coupled to a different stage (not shown). The twostages may be coupled by a common specimen handler. Many specimenhandlers such as wafer handlers are known in the art, and any specimenhandler may be used as the common specimen handler. In such anembodiment, the optical subsystem(s) may perform measurements on onespecimen while the cleaning subsystem is removing contaminants from adifferent specimen. In some such embodiments, the cleaning subsystem maybe contained within a different module than the optical subsystem(s).

In the embodiment shown in FIG. 17, the cleaning subsystem includes alaser-based cleaning subsystem. Such a cleaning subsystem is configuredto remove contaminants from localized area 328 on specimen 10. Thecleaning subsystem is preferably configured to remove contaminants froma portion of the specimen prior to the measurements by the opticalsubsystem(s). Particularly, the cleaning subsystem is configured toremove contaminants from a portion of the specimen prior to measurementsby VUV optical subsystem(s). Such a cleaning subsystem may beincorporated into all of the systems described herein.

One example of an appropriate laser-based cleaning subsystem isillustrated in U.S. patent application Ser. No. 10/056,271 entitled“Laser-Based Cleaning Device For Film Analysis Tool” filed on Jan. 23,2002, by Janik et al., which is incorporated by reference as if fullyset forth herein. As described in this patent application, cleaningsubsystem 324 includes energy beam source 330. During the cleaningoperation, a small portion of a contaminant layer (not shown) formed onthe upper surface of specimen 10 is removed. The exposed portion (i.e.,analysis area) of a layer (not shown) on the specimen is then analyzedduring the measurement operation. The position of stage 12 can beshifted relative to energy beam source 330 and the optical subsystem(s)to enable cleaning and analysis at multiple locations on specimen 10.

To perform a cleaning operation, energy beam source 330 directs energybeam 332 at spot 328 on the contaminant layer. Energy beam 332 isconfigured to remove a portion of the contaminant layer by heating thecontaminant layer directly or by heating the underlying portion of alayer on the specimen or the specimen. The portion of the contaminantlayer heated in this manner is eventually vaporized, thereby exposingthe underlying portion of a layer on the specimen or the specimen. Thisremoval process can be aided by other mechanisms besides heatingincluding the direct stimulation of the bonds between the contaminantlayer and the specimen by photons from energy beam 332.

Because the heating from energy beam source 330 is confined to alocalized area, the cleaning operation can be performed very rapidly,which minimizes any impact on analysis throughput. The potential fordamage to the underlying layer and/or substrate is minimal because onlya small portion of specimen 10 is heated. The risk of damage can befurther reduced by performing the cleaning operation on non-functionalregions of specimen 10 (e.g., regions such as scribe lines that will notbe part of the functional portion(s) of the final devices to be madefrom specimen 10).

The amount of the contaminant layer to be removed depends on themeasurement requirements of the optical subsystem(s). Modern thin filmanalysis tools generally require an analysis area of at least 20 μm×20μm. Therefore, at least a 20 μm×20 μm portion of the contaminant layerwould need to be removed for such systems. However, to ensure that theentire analysis area is uniformly cleaned, a larger portion of thecontaminant layer could be removed.

According to an embodiment, energy beam source 330 could include apulsed laser. For example, the contaminant layer could include a 5angstrom thick layer of water and organic materials (which is similar tocontamination layers often formed on modern thin film layers duringproduction). A number of pulses or even a single pulse from a 5–100μjoule laser having a 1–1000 ns pulse duration could then heat thedesired portion of the contaminant layer to between roughly 300° C. to1000° C., which is a temperature range sufficient to vaporize thatportion of the contaminant layer. According to another embodiment,energy beam source 330 could include a Q-switched laser delivering arelatively high peak power, such as a frequency-doubled or tripled YAG(yttrium aluminum garnet) laser operating at wavelengths of 532 nm or355 nm, respectively. According to another embodiment, other types ofpulsed lasers operating at different wavelengths might be used includingpulsed diode or alexandrite lasers. According to another embodiment, acontinuous laser, such as an argon-ion laser, could be externallymodulated (such as with an acousto-optic or electro-optic modulator) toproduce a pulse. According to another embodiment, energy beam source 330could including focusing optics (not shown) such as an optical fiber anda lens system configured to deliver a beam of the desired size andenergy to spot 328 from a remote location (i.e., the optical fiber couldtransmit energy beam 332 from a remote beam generator to spot 328).According to another embodiment, energy beam source 330 could include aflash lamp coupled to focusing optics (not shown) to direct the highintensity light to the desired area on the contaminant layer.

Once the cleaning operation is complete, the measurement operation canbe performed. Because specimen 10 does not need to be transferred to adifferent tool or process chamber, the measurement operation can beperformed immediately following the cleaning operation so that thechance of recontamination of the exposed portion (analysis area) of thespecimen is minimized. In this manner, a localized cleaning operationcan be efficiently combined with a measurement operation to ensureaccurate and repeatable analyses.

In the system shown in FIG. 17, energy beam source 330 and the opticalsubsystem(s) are not simultaneously focused at the same location onspecimen 10. Consequently, an analysis operation performed using theoptical subsystem(s) actually includes three steps: a cleaningoperation, a positioning operation, and a measurement operation. Duringthe cleaning operation, a small portion of the contaminant layer isremoved by the energy beam from energy beam source 330. Then, during thepositioning operation, specimen 10 is positioned such that light of oneor more of the optical subsystems is aligned with the portion of thespecimen exposed during the cleaning operation. The exposed portion ofthe specimen can then be analyzed by the optical subsystem(s) during themeasurement operation.

In an alternative embodiment, energy beam 332 and a sample beam from oneof the optical subsystems may be simultaneously directed atsubstantially the same location of specimen 10. For example, energy beam332 and the light from the optical subsystem disposed in the non-purgedenvironment may be simultaneously directed to substantially the samelocation on specimen 10. Therefore, the position of specimen 10 does nothave to be adjusted between cleaning and measurement operations by thisoptical subsystem. Therefore, the measurement operation can be performedimmediately after the cleaning operation to ensure that a newcontaminant layer is not reformed over the analysis area. It is to beunderstood that the cleaning subsystem illustrated in FIG. 17 may befurther configured as described in the above referenced patentapplication.

In a different embodiment shown in FIG. 18, the cleaning subsystemincludes a heat-based cleaning subsystem. Such a cleaning subsystem isconfigured to remove contaminants from substantially an entire surface334 of specimen 10. The cleaning subsystem is preferably configured toremove contaminants from a portion of the specimen prior to themeasurements by the optical subsystem(s). Particularly, the cleaningsubsystem is configured to remove contaminants from a portion of thespecimen prior to measurements by VUV optical subsystem(s). Such acleaning subsystem may be incorporated into all of the systems describedherein.

The cleaning subsystem illustrated in FIG. 18 is a conventional wafercleaning system 336 used to prepare specimen 10 for thin film analysisas described in U.S. Pat. No. 6,261,853 to Howell et al., U.S. Pat. No.6,519,045 to Kwon, and U.S. Pat. No. 6,624,393 to Howell et al., whichare incorporated by reference as if fully set forth herein. The cleaningsubsystem performs a bulk heating operation to remove contaminants froma specimen. Specimen 10 may include a thin film layer (not shown) formedon a silicon substrate (not shown) and a contaminant layer (not shown)formed on the surface of the thin film layer. Cleaning subsystem 336incorporates stage 338 that includes heating element 340. Heat generatedby heating element 340 is conducted through stage 338 into specimen 10thereby providing the heating required to vaporize the contaminantlayer. Heat exchanger 342 coupled to stage 338 captures excess heat fromheating element 340 thereby minimizing undesirable heating of thecleaning subsystem itself and the surrounding environment.

Stage 338 and stage 12 may be coupled by a common specimen handler (notshown). Many specimen handlers such as wafer handlers are known in theart, and any specimen handler may be used as the common specimenhandler. After cleaning by cleaning subsystem 336, the specimen may betransferred to stage 12 by the common specimen handler. In such anembodiment, the optical subsystem(s) may perform measurements on onespecimen while the cleaning subsystem is removing contaminants from adifferent specimen. In some such embodiments, the cleaning subsystem maybe contained within a different module of the system than the opticalsubsystem(s).

In an alternative embodiment, the cleaning subsystem may be configuredto heat the entire wafer in an oven (not shown) to a temperature ofabout 300° C. to vaporize any contaminants on the specimen. One exampleof a conventional oven-based wafer cleaning subsystem used to prepare awafer for thin film analysis is described in U.S. Pat. No. 6,325,078 toKamieniecki, which is incorporated by reference as if fully set forthherein. Such a cleaning subsystem may include multiple heating lamps(not shown), where thermal radiation from the heat lamps heats aspecimen to vaporize contaminants on the specimen.

The systems shown in FIGS. 17 and 18 may include more than one cleaningsubsystem. For example, a system may include a laser-based cleaningsubsystem and a heat-based or oven-based cleaning subsystem. Bothcleaning subsystems may be used to clean a specimen. The systems shownin FIGS. 17 and 18 may be further configured as described herein.

Current spectroscopic film measurement systems use a light source thathas a substantially continuous spectrum, generally with a small numberof peaks (e.g., 3 or less peaks) each riding on top of a substantiallycontinuous background. The spectrum is detected by an array detectorconsisting of an array of pixels and the spectrum is spread out alongthe array. It is necessary to know the relationship between the pixelnumber and the wavelength in order to process the data into a filmmeasurement. Wavelength calibration is performed occasionally with aseparate light source that has a spectrum of a moderate number of widelyseparated peaks (e.g., 5 or less separated peaks). The pixel numbers onwhich these peaks lie are used to fit a function of pixel number versuswavelength. This function is used to process the data. One or more ofthe peaks in the measurement lamp spectrum can be used to update thewavelength calibration in the interval between calibration processes.

There are, however, some disadvantages to the above describedmeasurement systems and methods. For example, the measurement spectrumcan drift on the detector in the interval between calibration processes,and can even move during a measurement. These drifts and movements causeerrors in the measurement because the relationship between wavelengthand detector position is not what it is assumed to be. The peaks in themeasurement source are difficult to track during a measurement orbetween measurement samples because the size and shape of the backgroundchanges during the measurement process for ellipsometers and changesbetween different samples for all spectral instruments. The imperfectpeak tracking causes measurement errors. Also, if only a small number ofpeaks are used to track the spectrum, changes in the spectrum's scale(e.g., stretching or shrinking) cannot be completely accounted for.

Therefore, systems and methods have been developed for improving theaccuracy and repeatability of a spectroscopic ellipsometer, polarimeter,reflectometer, or any other spectroscopic measurement system known inthe art that is configured for measurement of a specimen. The systemincludes an optical subsystem that is configured to perform measurementsof the specimen. In some embodiments, the measurements may includemeasurements of film properties. The optical subsystem may be configuredas a spectroscopic ellipsometer, a polarimeter, a reflectometer, or anyother spectroscopic measurement system described above or known in theart. Examples of ellipsometers and reflectometers are described above.In addition, examples of polarimeters are illustrated in U.S. Pat. No.6,177,995 to Compain et al., U.S. Pat. No. 6,181,421 to Aspnes et al.,U.S. Pat. No. 6,184,984 to Lee et al., U.S. Pat. No. 6,211,957 toErdogan et al., U.S. Pat. No. 6,535,286 to Green et al., U.S. Pat. No.6,591,024 to Westbrook, and U.S. Pat. No. 6,611,330 to Lee et al., allof which are incorporated by reference as if fully set forth herein. Theoptical subsystem may be further configured as described herein. Forexample, the optical subsystem may be disposed in a purged environmentcreated by a purging subsystem. In another example, the opticalsubsystem may be included in a system with a non-VUV optical subsystemand/or a cleaning subsystem, which may be configured as described above.

In one embodiment, the optical subsystem includes a light source that isused for the measurements. The light source preferably generates lighthaving a relatively large number of separated spectral peaks, with verylittle or substantially no continuous background. There are a largenumber of light sources that can be used to generate light having arelatively large number of separated spectral peaks, especially in thedeep VUV, extreme ultraviolet (EUV), and soft x-ray regions of thespectrum. Therefore, the optical subsystem may include a light sourcethat is configured to generate VUV light, EUV light, or soft x-rays. VUVlight is defined above. EUV light is generally defined as light havingwavelengths between about 100 nm and about 10 nm. Soft x-rays aregenerally defined as having wavelengths between about 10 nm and about0.5 nm. In addition, the optical subsystem, in some embodiments, mayinclude a light source that is configured to generate light havingwavelengths of less than about 200 nm.

In one example, Penning or hollow-cathode discharges with various metalsor alloys of metal used in the cathodes can be included in the opticalsubsystem. A spectrum of a Penning discharge with a magnesium (Mg)cathode and neon (Ne) buffer gas is shown in FIG. 19. As shown in FIG.19, which is taken from the book “Vacuum Ultraviolet Spectroscopy I”, byJames A. Sampson, p. 77, the spectrum of the Penning discharge includes16 separated spectral peaks. However, the light source may provide adifferent number of separated spectral peaks. Preferably, the lightsource provides about 5 or more separated spectral peaks and morepreferably about 10 or more separated spectral peaks. In anotherexample, in the spectrum between about 160 nm and 90 nm, the spectrum ofH₂ includes a relatively large number of narrow peaks. Discharges inmixtures of noble gases can also be used as the light source. Highintensity pulsed laser harmonic generation and other nonlinear processescan also be used to generate large numbers of sharp spectral peaks.

Examples of light sources that can be used to generate soft x-raysinclude a 1.064 μm neodymium YAG, laser which produces a xenon gasplasma that outputs soft x-ray radiation. Alternatively, another softx-ray source may be used as the light source in the optical subsystemsuch as a synchrotron, discharge pumped x-ray lasers, an electron-beamdriven radiation source device, or a radiation source based on highharmonic generation based on femto-second laser pulses. Other examplesof light sources that can be used to generate soft x-rays are describedin U.S. Pat. No. 6,522,717 to Murakami et al., which is incorporated byreference as if fully set forth herein. Any of the above-described lightsources may be included in any of the optical subsystems describedherein. The configuration of the optical subsystem may be altereddepending on the selected light source. However, such design changes arewell within the skill of one of ordinary skill in the art, and thereforewill not be described further herein.

The optical subsystem also includes one or more detectors that areconfigured to detect light returned from the specimen. In someembodiments, the detector(s) may preferably be array detectors. Thereare several types of array detectors that can be used in the opticalsubsystem including, but not limited to, bare CCDs, photodiode arrays,microchannel plates (MCPs), and phosphor plates fiber-coupled to CCDs.Signals generated by the detector(s) may be processed by a processor andoptionally one or more electronic components that may couple thedetector to the processor. The processor may be further configured asdescribed above. In addition, the processor may be coupled to theoptical subsystem as described above.

When the measurement data is processed, the data may be processed bybinning the signal into peaks. For example, a software algorithm can beused to partition the data into individual peaks. Peaks that span morethan one pixel have their signal summed over the spanned pixels. Thesignal from each peak is considered to come from a single wavelength(e.g., the known wavelength of the peak center). In this way, the signalis processed using peaks as the fundamental unit of data, instead ofpixels as in the current methods and systems.

The processor described above may perform these functions. For example,the processor may be configured to process data generated by the opticalsubsystem by partitioning the data into individual peaks spaced apartacross a wavelength spectrum. The individual peaks correspond to theseparated spectral peaks of the light generated by the light source. Inaddition, the processor may be configured to process data generated bythe optical subsystem by summing signals for a peak in light detected bythe optical subsystem if the peak spans more than one pixel on adetector of the optical subsystem. In a further example, the processormay be configured to process data generated by the optical subsystem byconsidering a signal from a peak in light detected by the opticalsubsystem as having a single wavelength. The single wavelength mayinclude a known wavelength of a center of the peak. The processor may befurther configured as described above.

In other embodiments, program instructions, which may be included in acarrier medium, that are executable on a computer system may beconfigured to perform the above described functions. For example, in oneembodiment, the program instructions are executable on the computersystem to analyze data generated by a detector of the optical subsystemby partitioning the data into individual peaks spaced apart across awavelength spectrum. The individual peaks correspond to separatespectral peaks in light generated by the light source of the opticalsubsystem. In another embodiment, the program instructions areexecutable on the computer system to analyze the data by summing signalsfor one or more of the individual peaks if the individual peak(s) spanmore than one pixel on the detector. In a further embodiment, theprogram instructions are executable on the computer system to analyzethe data by considering a signal from one or more of the individualpeaks as having a single wavelength. The single wavelength includes aknown wavelength of a center of the individual peak(s). The programinstructions and the carrier medium may be further configured asdescribed above. In addition, separate, different program instructionsmay be executable to perform the functions described above. The separateprogram instructions may or may not be included in one set of programinstructions.

In one particular example, it may be desired to measure a substantiallythin film. An EUV reflectometer containing a Penning discharge lightsource with a Mg cathode is used to collect reflectance data over aspectral range of about 12 nm to about 28 nm. The signal generated bythe optical subsystem is binned into spectral peaks, and each peak isassigned its known central wavelength. The data may be further processedas described above.

The system and carrier medium embodiments described above have severaladvantages over the currently used systems and methods. For example, thewavelength calibration of the detector is automatically maintained atall times and is updated at each exposure of the detector. In addition,spectrum shift, drift, stretching, and shrinking are more completelyaccounted for than in the current methods and systems. Furthermore, themethods and systems described above would improve film measurementcapability at substantially short wavelengths (e.g., less than about 200nm). In particular, partitioning of the data as described abovemaintains a wavelength calibration of the detector, corrects forspectrum shift, drift, stretching, shrinking, or a combination thereofas the detector, and/or increases an accuracy of the measurements of theoptical subsystem that are performed as a function of wavelength.

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. For example, systems and methods for measurement of aspecimen with vacuum ultraviolet light are provided. Accordingly, thisdescription is to be construed as illustrative only and is for thepurpose of teaching those skilled in the art the general manner ofcarrying out the invention. It is to be understood that the forms of theinvention shown and described herein are to be taken as the presentlypreferred embodiments. Elements and materials may be substituted forthose illustrated and described herein, parts and processes may bereversed, and certain features of the invention may be utilizedindependently, all as would be apparent to one skilled in the art afterhaving the benefit of this description of the invention. Changes may bemade in the elements described herein without departing from the spiritand scope of the invention as described in the following claims.

1. A system configured for measurement of a specimen, comprising: anoptical subsystem configured to perform measurements of the specimenusing a light source configured to generate light having a relativelylarge number of separated spectral peaks with substantially nocontinuous background; and a processor configured to process measurementdata generated by the optical subsystem using individual peaks in themeasurement data as the fundamental unit of data, wherein the individualpeaks correspond to the separated spectral peaks of the light generatedby the light source.
 2. The system of claim 1, wherein the light haswavelengths of less than about 200 nm.
 3. The system of claim 1, whereinthe light comprises vacuum ultraviolet light.
 4. The system of claim 1,wherein the light comprises extreme ultraviolet light.
 5. The system ofclaim 1, wherein the light comprises soft x-rays.
 6. The system of claim1, wherein the relatively large number of the separated spectral peakscomprises about 10 or more separated spectral peaks.
 7. The system ofclaim 1, wherein the optical subsystem is further configured as aspectroscopic ellipsometer, a polarimeter, or a reflectometer, andwherein the measurements comprise measurements of thin film properties.8. The system of claim 1, wherein the optical subsystem furthercomprises an array detector configured to detect light returned from thespecimen.
 9. The system of claim 1, wherein the processor is furtherconfigured to process the measurement data generated by the opticalsubsystem by considering a signal from a peak in light detected by theoptical subsystem as having a single wavelength, and wherein the singlewavelength comprises a known wavelength of a center of the peak.
 10. Thesystem of claim 1, wherein the processor is further configured toprocess the measurement data generated by the optical subsystem bysumming signals for a peak in light detected by the optical subsystem ifthe peak spans more than one pixel on a detector of the opticalsubsystem.
 11. The system of claim 1, wherein the processor is furtherconfigured to process the measurement data generated by the opticalsubsystem by partitioning the measurement data into the individual peaksspaced apart across a wavelength spectrum.
 12. A carrier medium,comprising program instructions executable on a computer system toanalyze measurement data generated by a detector of an optical subsystemby partitioning the data into individual peaks spaced apart across awavelength spectrum and using the individual peaks as the fundamentalunit of data, wherein the individual peaks correspond to a relativelylarge number of separated spectral peaks with substantially nocontinuous background in light generated by a light source of theoptical subsystem, and wherein the optical subsystem is configured toperform measurements of a specimen using the light generated by thelight source.
 13. The carrier medium of claim 12, wherein the programinstructions are further executable on the computer system to analyzethe measurement data by summing signals for one or more of theindividual peaks if the one or more of the individual peaks span morethan one pixel on the detector.
 14. The carrier medium of claim 12,wherein the program instructions are further executable on the computersystem to analyze the measurement data by considering a signal from oneor more of the individual peaks as having a single wavelength, andwherein the single wavelength comprises a known wavelength of a centerof the one or more of the individual peaks.
 15. The carrier medium ofclaim 12, wherein the partitioning of the measurement data maintains awavelength calibration of the detector.
 16. The carrier medium of claim12, wherein the partitioning of the measurement data increases anaccuracy of the measurements of the optical subsystem that are performedas a function of wavelength.
 17. The carrier medium of claim 12, whereinthe partitioning of the measurement data corrects for spectrum shift,drift, stretching, shrinking, or a combination thereof at the detector.18. The carrier medium of claim 12, wherein the optical subsystem isfurther configured as a spectroscopic ellipsometer, a polarimeter, or areflectometer, and wherein the measurements comprise measurements ofthin film properties.
 19. The carrier medium of claim 12, wherein thelight has wavelengths of less than about 200 nm.
 20. The carrier mediumof claim 12, wherein the light comprises vacuum ultraviolet light. 21.The carrier medium of claim 12, wherein the light comprises extremeultraviolet light.
 22. The carrier medium of claim 12, wherein the lightcomprises soft x-rays.